OVERHEAD POWFR LINES ELEMENTARY DESIGN AND CALCULATIONS BY CAPTAIN W. MORECOMBE, R.E. B.Sc. (ENG.), A.M.I.E.E. LONDON CHAPMAN & HALL, LTD. 11 HENRIETTA ST., W.C. 2 1929 3^95 POINTED IN GREAT BRITAIN I'JY TMK ABERDEEN UNIVERSITY I'RKSS ABKRDKRN, SCOTLAND IX CONTENTS. Introduction . . . . . Index to Working Tables x Index to Working Curves xi CHAP. I. Electrical Considerations ....... 1 II. Copper Conductors, Sag and Stress Calculations . . .18 III. Conductor Arrangement, Clearances and Spacing ... 40 IV. Insulators .......... 55 V. Cross Arms and Insulator Brackets 70 VI. Simple Wood Supporting Poles 94= VII. Compound Wood Poles . 115 VIII. Iron and Ferro-concrete Poles . . . . . .135 IX. Angles and Terminals 163 X. Conductors other than Copper . . . . . .183 XI. Safety Precautions 193 APPENDIX, I. E.C. Regulations for O.H. Lines 201 II. Information to be furnished to the E.C. when proposing to erect O.H. Lines 214 III. P.O. Regulations affecting design of O.H. Power Lines in the neighbourhood of Telegra|)h and Telephone Lines H.V. . 219 IV. P.O. Regulations affecting design of O.H. Power Lines in the neighbourhood of Telegraph and Telephone Lines L.V. . 222 General Index: .......... 233 INDEX TO WORKING TABLES. PAGE I. Particulars of Hard Drawn. Copper Conductors ... 4 II. Erection Sags, H.V. Lines, H.D. Copper Conductors . . 24 III. Erection Tensions, H.V. Lines, H.D. Copper Conductors . 25 IV. Erection Sags, L.V. Lines, H.D. Copper Conductors . . 26 V. Erection Tensions, H.V. Lines, H.D. Copper Conductors . 27 VI. Maximum Angles for H.V. Pin Insulators . . . .61 VII. Lengths of Binding Wire required ..... 66 VIII. Particulars of Steel Sections . . . . . .71 IX. Ultimate Stresses, etc., of Timber and Steel ... 73 X. Particulars of Bolts . . . . . . . .75 XI. Basic Loading Values on Standard Span Lengths . . 78 XII. Particxilars of Standard Creosoted Fir Poles ... 97 XIII. Costs of Supports for Various Span Lengths . . . 102 XIV. Suggested Standard Span Lengths for H.V. Lines . . 106 XV. Particulars of Channel Sections . . . . .146 XVI. Particulars of Stay Wires and Stay Rods . . . 170 XVII. Particulars of Various Soils ...... 179 XVIII. Particulars of Various Line Conductor Materials , . .184 XIX. Comparison: Aluminium and Copper Conductors . .186 XX. Comparative Cost : Aluminium and Copper Conductors . 1 87 XXI. Particulars of Galvanised Steel Conductors .... 189 XXII. Sags and Tensions of Galvanised Steel Conductors . . 190 XXIII. Illustrating Economy duo to Steel Conductors . . . 191 XI INDEX TO WORKING CURVES. FIG. 6. Voltage Drop per Mile at Various Loads (10 000 volts) . 7. Energy Loss per Mile at Various Loads (10 000 volts) . 8. Percentage Energy Loss per Mile at Various Loads (10 000 volts) 13. Erection Sags, H.V. Lines, -05 sq. in. Copper 14. Erection Tensions, H.V. Lines, -05 sq. fh. Copper 15. Sags of Copper Conductors at 122 F., H.V. Lines 16. Sags of Copper Conductors at 122 F., L.V. Lines . 17. Sags of Copper Conductors at 62 F., H.V. Lines . "IS. Sags of Copper Conductors at 62 F., L.V. Lines . 25. Measurement of Sags by Swings ...... f) 41. Values of 2 sin ~ 49. Strength of Single Poles 51. Chart for Selecting Size of Pole required 55. Buried Depth of Single Wood Poles .... 65. Strength of Rutter Poles .66. Strength of " A " Poles 67. Strength of " H " Poles 91. Strength of Anchorages ...... :} :} PAGE 11 12 13 29 30 . 43 . 60 . 96 to face 101 . 110 132 133 181 ELECTRICAL CONSIDERATIONS 3 If R t , L t) X t and Z t denote the RESISTANCE INDUCTANCE, REAC- TANCE AND IMPEDANCE respectively per 1 000 yards of single con- ductor, then the a INDUCTANCE, L t = ( 421 log- + - 0457 /z) mH. REACTANCE, X t = 27rfL t . /, the frequency, is taken as 50 in all calculations in this chapter. Since the reactance varies with the frequency a correction will have to- be applied to all figures given for voltage drop if the system fre- quency is other than 50 IMPEDANCE, Z t = x/^ 2 + X t \ JO 20 30 40 50 60 Inches Distance between Conductors FIG. 1. Variation of reactance with spacing. To simplify matters attention will be confined mainly to the consideration of eight standard sizes of Hard Drawn Copper Conduc- tors, working particulars of which are given in Table I. Values for intermediate sizes can be deduced with sufficient accuracy by inter- polation. (The full range of British Standard Solid and Stranded Hard Drawn Copper Conductors is given in B.S. Spec. No. 1.25.) The use of other conductor materials is discussed in Chapter X., page 183. The values ofR t in Table I. are taken from the standard specifi- cation, and those of X t have been evaluated from the formula given above for a spacing of 3 feet and a frequency of 50 cycles per second. Assuming a constant value for the reactance greatly simplifies OVERHEAD POWER LINES i Overall Biameter Wire + Ice, Weight of Wire -f Ice. Wei OOOOCOrHOOOt- o 10 -* oo I-H co o co //Jl/8 wd cutS mn oooi ioa oot 4 a! o Hi ^,0 si! " OOCOOt'-JO'^COCO CO CM CO l> 00 CO 00 Oi OOr-H^COCClO-^CN OOOOiOrHrHCMCC CO r-4 Oi r TjH io ^ O CM OS 00 O OS rH O sT ooo ^S^nS^n - - Rj TJ 4 CO 00 ^H r-4 OS CM C 571 rH (N C<l T 0000000 o o o o vo o o THCOCR>CqcOCOOCNI rH O 03 CO 0? 00 00 CD i# I <M T-H OV XO CO ioaso-ioo Ot-f-HOO O < CM CO ... N^ COCOCOCOCOCOCOCvI tipx& OOOT -880.10 ^ (MrHCOOCOlO COW U1SI3AO CO rH CO O CO <M "* o as rH G<J (N i O O O ' 'O.>O O r-i rH 1 O4 cocNcoc-ocacoooi o COCOOirHOCOOlr- "XS F-Hr-Hr-tCOCO^rHl7l co CN cc i> co co co > "^' 311 p^. ELECTRICAL CONSIDERATIONS the calculations and it will be clear from Fig. 1 that no serious erro is involved in so doing since in the short span construction con sidered in this book the spacing will never exceed 5 feet. A copper conductor of diameter 0- 162 inches (0- 0201 sq. in.) i the smallest allowed by the E.G. Regulations for O.TL lines in thi country, and the stranded conductor 7/ * 193 (0-2 sq. in.) is the larges which is, as a rule, justifiable economically. " This will be clear fron Fig. 2 which shows how the values of R, X and Z vary with th< cross-section. There is obviously little advantage in using conductor; larger than 0-15 to reduce voltage drop, owing to the swamping effec 1-0 I I V -05 -JO -15 -20 -25 Cro?s Section, in Square Inches PIG. 2. Variation of R, X and Z with, cross-section. of the reactance. This limitation does not, of course, apply to under ground cables in which the conductors are much closer together. Calculation of Voltage Regulation. The vector diagran in Fig. 3 will be found useful in enabling us to visualise how th< various factors involved are related to one another. 01 = current vector = vector of reference. Cos <f) = power factor of load. OE = transmission voltage. 0V delivery voltage. OR = VA = resistance drop (RI) parallel to 01. OX AE = reactance drop (XI) perpendicular to 01. OZ = VE = impedance drop. PE = OE 0V = LINE VOLTAGE DROP. OVERHEAD POWER LINES It is important not to confuse the impedance drop (VE) with the true LINE VOLTAGE DROP (PE), which is the arithmetical difference between OE and OF. PE The VOLTAGE REGULATION is defined as the ratio PE ov The PERCENTAGE VOLTAGE REGULATION = ^= x 100 per cent. Vectors are shown for power factors 0-8 lagging, unity and 0-8 leading. 0V = 10 units, and to the same scale RI = 2 and XI = 'B K,W. Constant O.V. [Constant] '10,000 Volts C05 M ( R -'- e Lx.i, - c.,,...(; : " 2,000 v/olts 3,000 ^. 2,500 3,750 OE* , OV- 10,000 Volf* Fia. 3. Transmission line vector diagram. 3 units at unity P.F., the values at 0-8 P.P. are therefore 2-5 and 3-75 respectively. For clearness, these values of RI and XI are much exaggerated. In practice the impedance drop seldom exceeds 10 % of the delivery voltage. From the geometry of the figure we have 0$ 2 = (OF cos d> 4- * + (0V sin $ + XI)*. If the current leads on the voltage, < must be taken as negative, ELECTRICAL CONSIDERATIONS 7 in which case, although cos </> remains positive, sin <j> becomes nega- tive. This expression is rather cumbersome for general use, but the following simpler formula gives results sufficiently accurate for most practical purposes with a lagging power factor. OE = OF + RI cos <f> + XI sin <. This is deduced as follows : - Draw EB perpendicular to 0V produced. Then VB is approxi- mately equal to PE. But VB = VA cos < + AE sin </> RI cos </> + XI sin <f>. The results obtained by this simpler formula are exactly correct for one value of </> only with each size of conductor, viz., but over the usual range of lagging power factors the error intro- duced by its use is always inappreciable. For leading power factors, however, it is necessary to use the more exact formula with sin <f> negative. If R and X are values for a single conductor, then the total line voltage drop will be as follows : SINGLE-PHASE : V.D. = 2(RI cos & + XI sin <f>) THREE-PHASE : V.D. = ^/Z(RI cos </> + XI sin <) In a simple length of 3-phase line without branches, Watts delivered = W = \/3VI cos $ W /.I = . 7 . cos <-/>" If this value of I is substituted in the equation for voltage drop we have *J~*> W V.D. = / - v ' (R cos d> + X sin </>) ^ n OVERHEAD POWER LINES Assuming F= 100, 7= 10000, /== 50, balanced load and an equilateral triangle conductor spacing of 3 feet, we get 100000 X l-76 /T > , , 10000 V.D. per mile = The curves in Fig. 4 have been plotted to illustrate the ADVERSE 40 35 -5 -9 -9 1-0 : d -8 Leading Lagging Load Power Factor FIG. 4. Volts drop per 100 K.W. per mile. 10 000 volts, 3-phase, 50 cycles, 3 ft. spacing. Showing effect of power factor. EFFECT OF Low LAGGING POWER FACTOR ON VOLTAGE REGULATION. The values for B t and X t being per 1 000 yards of single conductor, taken from Table I. ; the effect will be seen to be more serious with the larger conductors. It will be noted, however, that the introduction of a leading current at the delivery end of the line by means of condensers or ELECTRICAL CONSIDERATIONS 9 over-excited synchronous machinery will reduce the line voltage drop. In certain circumstances it may pay to put in apparatus to take sufficient leading current to reduce the line drop to zero or even to produce a rise of volts in the line. This will be seen from Fig. 4 to be quite practicable with the larger conductors. woo 900 1-0 9 -8 -7 '6 Load Power Factor FIG. 5. 1.0 a o. rower race or -Energy loss per 100 K.W. per mile. 10 000 volts, 3-phase, 50 cycle Showing effect of power factor. Energy Loss in Conductors. Although the voltage regula- tion will usually be the determining factor in the lines under con- sideration, it is desirable to know approximately the energy loss that will occur in the conductors. The percentage energy loss is generally somewhat greater than the percentage voltage drop, the two values being approximately equal when the load power factor is unity. 10 OVERHEAD POWER LINES The Total Energy Loss in a 3-phase system == 3 . P . R / W \ 2 __ r>{ YY \ 7? = & I T= - T ) it . V . cos cf>/ 2 R V J * cos 2 <f>' R being as before the resistance of a single conductor. The energy loss is therefore inversely proportional to the square of the power factor. Taking the same load conditions as those assumed in Fig. 4, we get : ENERGY Loss PER MILE 10 000 >< >< - = 176 - -, watts. The curves in Fig. 5 have been plotted from this formula to illustrate the ADVERSE EFFECT OF Low POWER FACTOR on the efficiency of transmission. Power Factor Correction. The importance of high power factor will be realised from the above, the cost of the conductors depending upon K.V.A. and not K.W. An equitable charge to power consumers can, therefore, only be made on a K.V.A. basis. Sxich a tariff will usually influence them to install apparatus for power factor correction with advantage to all concerned. Working Curves for V.D. and Energy Loss. A number of charts will now be given to facilitate the selection of the nearest standard conductor to satisfy a given set of conditions. It will be sufficient for practical purposes to design for a load power factor of 0-8. This should usually give a safe margin, as during the lighting hours, when the problem of voltage regulation is most acute, the P.F. should be much higher than this. A delivery voltage of 10 000 will be assumed, i.e. the current taken in the calculations will be the largest experienced per K.V.A. on a standard 11 000 volt system, Voltage Drop per Mile (Fig. 6). Since the V.D. is pro- portional to the current, and the current (at constant P.F.) is proportional to the K.V.A., the curve connecting V.D. and K.V.A. will be a straight line. ELECTRICAL CONSIDERATIONS 11 If the load is 500 K.WL at 0-8 P.F.,. then V.D. per 1 000 yards = V3~. I . (R t cos $ + Z t sin <) and V.D. per mile _ AT 500000 X 1'76 VoX 7= - X ( oA t + A/3 X 10 000 X '8 V R t and Z t being values per 1 000 yards, obtained from Table I. E.g. for -193 conductor, R t = -SQ and ^=-355 .-. V.D. per mile = 110 (-8 X -86 + -6 X -355) = 99 volts. 200 780 160 140 120 to ^ X 30 60 40 20 /6 1-6 7-2 -^ - -er -.4- / 7 7 7<?0 2^ ^ 4^0 500 600 700 K. \M. Delivered FIG. G. Voltage drop per mile. Balanced load. 10000 volts, 0-8 P.F., 3-; 50 cycles, 3 ft. spacing. 12 OVERHEAD POWER LINES This point is plotted in Fig. 6, and a straight line drawn through it from the origin. Straight lines for the other seven selected con- ductors are drawn similarly. The Percentage Voltage Drop can be read off the same chart. 300 20 40 60 80 - ,100 Delivered 100 - 200 300 400 500 600 700 K. WA Delivered Energy loss per mile. Balanced load. 10000 volts, 0'8 P.F., 3 -phase, 50 cycles. ELECTRICAL CONSIDERATIONS ENERGY Loss PER MILE (Fig. 7). If K denotes the livered, then T ___ x/3 X 10 000 X -8 8^3 ,'. Energy loss per mile = 3 . 1 2 . 1'76 . R t _ 3 . K* . 1-76 . R t ' " 192 = -0275 .R t .K*i e.g. for -193 conductor R t == -86 ohm ; == J^ amps a t 0-8 P.F. 1-8 1-6 1-4- 1-2 -u c: * \-<& 100 200 300 400 500 600 700 K.\^f(. Delivered \ __% Energy loss per mile. Balanced load, 10 000 volts, 0'8 P 3 -phase, 50 cycles. 14 OVERHEAD POWER LINES and if K = 100 K.V.A., energy loss per mile = -0275 X -86 X 100 2 = 236-5 watts and so on. Proceeding in this way we are able to plot the curves shown in Fig. 7. % ENERGY Loss PER MILE (Fig. 8). The Energy Loss per mile = -0275 R t K z watts. The Energy Delivered = 1 000 . K . cos <j> watts. 0275 .R+.K* = -00344. R t .K.%. ,.- This is also the equation to a straight line, therefore the % loss need only be calculated for one value of the load with each size of conductor. Using -193 conductor for 500 K.V.A. % Energy loss per mile = -00344, -86 . 500 = 148 %. This point is plotted in Fig. 8 and a straight line drawn through it from the origin. Curves for the other conductors are drawn in a similar manner. % V.D. AND ENERGY Loss AT OTHER VOLTAGES. <V VD , /o VJJ. For a given conductor and load P.F. V.D. oc I. If the K.V.A. is constant, I oc =. .-. V.D. oc and the % V.D. oc -^. % ENERGY Loss. For a given conductor, energy loss oc I 2 . If the K.V.A. is constant, and also the P.F., / oc =^, .-. / Energy loss oc =r 2 . We see, therefore, that "both % voltage drop and the % energy loss are inversely proportional to the square of the working voltage. At 6 000 volts', the values are ( j = 2-78 times the value at 10 000 volts and at 20 000 volts, the values are only one quarter as great. ELECTRICAL CONSIDERATIONS '17 station is of the order 50 to 100 K.V.A. for distribution to small consumers, feeding an area of a few hundred yaids' radius only. Consumers taking 50 K.V.A. or more should be given a H V supply direct. The principles already given for deciding the size .of conductor for H.V. lines apply equally to L.V. lines, but in new work the stan- dard 400/230 volts, 4 wire, 3-phase system should always be adopted. The neutral conductor should be equal in size to the line con- ductors, ^the neutral drop being always appreciable owing to the impossibility of maintaining a balanced load. 18 CHAPTER II. CONDUCTORS, SAG AND STEESS CALCULATIONS. THE true shape of the curve formed by a uniformly loaded wire strung between two supports is a Catenary, but when the ratio of span (L) to sag (D) is large (say) greater than 10 to 1 , it is sufficiently accurate for practical purposes to assume that it is a parabola 3 and the calculations are thereby much simplified. The fundamental span formulas are (Kg. 10) *-lr -. m l = L ,ao 2 9 sag at middle of span in feet. L = length of span in feet. = length of wire in span in feet. W = total loading of wire in pounds per foot run, r = horizontal tension in wire at centre of span in pounds (assumed uniform throughout the span). It is unnecessary to distinguish between I and L when calculating roltage drop and measuring up length of wire required* The actual tension in the wire is not exactly uniform throughout ie span, but gradually increases towards the supports where it has ;he maximum value y 2* + (^ . For the ratios of span length ;o sag recommended in this book it will be found that the maximum CONDUCTORS, SAG AND STRESS CALCULATIONS 19 tension is only a fraction of 1 % greater than tlie horizontal tension, and. therefore the discrepancy may be safely neglected. The total loading W, which produces the tension T, comprises : 1. The dead weight of the wire. 2. The weight of any ice, sleet or snow that may cling to the wire. 3. Wind pressure. Basic Conditions of Stress. In order to ensure that a reasonable factor of safety is allowed for in erecting the conductors, the Electricity Commissioners have laid down the Basic Loading Conditions to be assumed (see E.G. Regulations, Appendix I.). For High Voltage Lines (above 650 volts) these are as follows : Temperature. 22 F. Ice Loading. f inch radial thickness. Wind Pressure.- 8 Ib. per square foot horizontally at right angles to the line on whole diameter of ice covered con- ductor. Factor of Safety. 2, based on the breaking stress given in B.S. Spec. 125 for hard drawn copper conductors. For Low Voltage Lines. In order to permit a lower real factor of safety the hypothetical ice loading is reduced to ^y inch. All otlier basic loading conditions are the same as for high voltage lines. It may be remarked that the above figures for temperature, ice loading and wind pressure are supposed to represent the worst weather conditions likely to be experienced simultaneously in Great Britain, and are legally applicable in this country only. In tropical countries it will be possible to work with smaller sags owing to the absence of ice, but it must not be overlooked that the wind pressures experienced are frequently greater than in this country. Minimum Sag under Basic Loading Conditions.- The starting-point in the sag and stress calculations, is to find the mini- rawm sag which must be allowed to ensure that the safe maximum stress is not exceeded under the worst loading conditions. Constants for the eight standard conductors under consideration are given in Table I., page 4, the notation being as follows (see Fig. 11):- 20 OVERHEAD POWER LINES d diameter of conductor in inches (B.S. Spec. 125). t ^ = thickness of ice coating in inches (f inch for H.V., yV inch for L.V. lines). d = <? -f- 2t = diameter of ice-covered conductor in inches. w = weight of conductor in pounds per foot run (B.S. Spec. 125). ID i = I'25t(d Q + t) = weight of ice in pounds per foot run. This assumes weight of ice as 57-3 Ib. per cubic foot ; B.C. Regulations specify 57 Ib. ID W Q -j- Wf - total vertical load in pounds. Pd 8d 2 .,,.,. <p =L= -=:= -d = wind load in pounds per foot run (acting 12 1 2 o horizontally). W = */w*' -j- p z = total load on conductor in pounds per foot ran. Pro. 11. Sag and Tension for Erection Purposes. The weather conditions at the time of erection are very different from those producing the maximum stress and it is therefore necessary to deter- mine what tension and sag should be allowed when the wire is erected in still air at various temperatures in order that the maximum stress shall not exceed the safe limit under the basic loading conditions. Perhaps it should be explained that considerations of pole sixes and conductor spacings demand generally that the sag of the conductors should be as small as possible. The smaller the sag the lower the F of 8, hence the necessity for a legal limit. The calculations are somewhat involved since expansion due to increase of temperature is accompanied by contraction due to elas- ticity. The simplest form of these calculations will now be given. Critical Temperature. Starting from 22 F. with ice loading, as the temperature rises, the wire elongates and the sag increases up CONDUCTORS, SAG AND STRESS CALCULATIONS 21 to 32 F., when the -reduction in loading due to the melting of the ice results in contraction due to elasticity. As the temperature continues to rise the sag increases again until at a certain tempera- ture the sag has again the sa.me value that it had originally under the basic loading conditions at 22 F. The temperature at which this' occurs is called the " Critical Temperature," the conception being of great use in solving sag and stress problems in overhead line con- ductors. Let D m , 8 m and T m denote the sag, stress and tensile load re- spectively under the basic loading conditions and D c , S c and T c the values at the critical temperature. wra w ra Then D = D = = ^- J-JJOU J^ J - f m c/m QT\ > OJ-m CJ-c T -*- !f. T inrl <? !fi <? -* o TI/ -* m> "J-iii fJo ' TIT '-'w Let 6 e denote the critical temperature, K the coefficient of linear expansion per deg. F. = 9-222 X 10-(B.S.S. 125), and ' M the modulus of elasticity in pounds per square inch = 18 X 10 G (B.S.S. 125). Then the elongation due to temp, rise = I . K , (# 22) I and the contraction due to elasticity = (S m S c ) p. These values are equal ; therefore, equating one to the other we get -i ""() WJ ~ 166V 1 "" W. from which 9 e can be calculated . For example, substituting the known values for -05 square inch copper from Table I. for II. V. lines we get 29 200 / -2 .-. C = 136 + 22= 158 F, Values of 6 C are given in Table I, page 4. To Find Sag and Tension to be Allowed on Erection. Decrease in Length due to fall in temp, from O c to some other temp. 9, = 1 C K(B - 6), 1 being the length of wire in span at the critical temp. 9 OVERHEAD POWER LINES w L 2 For the unloaded wire in still air, T.D. = -~ which is constant for o one particular length of span and size of conductor, T.D. = T C D C and S.D. = S C D C T, D and S being values for temp. 9, Increase in length due to increased tension If Z is the length of wire at temp. 6, then 8 - ' 2 This must be equal to me difference between the increase in length due to elastic extension and the decrease in length due to thermal contraction. ... (D e > - V) = lJSi(B, -6)- ? - l. At tMs stage no appreciable error is introduced by substituting L for l c , therefore equation may be written : - D \KM from which D can be calculated for any values of and i. The first rt fl I 7" ^ step is to find S c which equals Q . /S c is independent of the o . Jj c a length of span since D c oc L 2 . Substituting known values for *05 square inch copper (3/-147) (H.V. loading) from Table I., page 4, we get CONDUCTORS, SAG AND STRESS CALCULATIONS The following table can now be prepared : Span, 8 8Z>t 3 S C D G Se feet. DC. .Do 2 - s/a 3 ' 3A'L 2 " KM' KM 100 755 57 28-92 lft-5 30-15 39-94 150 1-70 2-89 12-85 37-1 67-8 39-94 200 3-02 9-1 7-23 65-9 120-6 39-94 250 4-72 22-3 4-63 103-0 188-4 39-94 300 6-79 46-0 3-21 148-3 271-4 39-94 1 -Z 3 4- Sag* In Feet FIG. 12. Graphic method of solving cubic equations. 3/'147 copper conductor, H.V. loading, 250 ft. span. Substituting all the known values in equation for D we liave for a 250 feet span. 158 - 6 = 103 - 4-63D 2 + ^^ - 39-94 1884 D D + 94-94, 188-4 e.g. if 9 = 62 we have 4-63D 2 ~-~ + 32-94 =-. i,e. D 3 = 40-8 7-12D and D = 2-765 feet. OVERHEAD POWER LINES TABLE IT.- Hard Drawn Copper Conductors. Sags in Still Ait for Erection Purposes. High Voltage Lines (f-w. Ice). Span in Feet. Temp. Paht. 162. 193. 3/-H7. 8/-18. 7/-136. 7/-166. 7/-193. Sq. ins. 02061 02926 05 075 10 15 20 feet, feet. fpet. feet. feet. feet. feet. 150 122 2-25 1-57 1-19 1-06 0-92 0-92 0-92 82 l-OO 1-02 0-78 0-71 0-63 0-62 0-63 02 1-37 0-83 0-65 0-60 0-53 0-53 0-54 42 1-10 0-08 0-55 0-51 0-46 0-46 0-47 22 0-87 o-r>7 0-48 0-44 0-44 0-40 0-41 22 3-06 2-34 1-70 1-33 1-035 5-5.5 -765 200 122 4-62 3-35 2-38 1-99 1-66 1-62 1-58 82 4-04 2-04 1-70 1-40 1-18 1-15 1-12 02 3-72 2-26 1-42 1-18 1-01 0-98 0-97 42 3-40 1-92 1-20 1-01 0-87 0-80 0-85 22 3-04 1-61 1-02 0-88 0-77 0-75 0-75 22 5-U 4-18 3-02 2-36 1-84 1-52 1-36 250 122 7-00 5-70 4-02 3-29 2-60 2-51 2-45 82 7-14 4-90 3-18 2-49 1-97 1-95 1-80 02 ($-83 4-00 2-77 2-14 1-70 1-57 1-57 42 0-5.1 4-19 2-39 1-83 . 1-47 1-39 1-37 22 0-22 3-78 2-05 1-59 1-29 1-23 1-21 22 8-50 6-53 4-72 3-69 2-873 2-38 2-12 300 122 11-40 8-58 6-05 4-86 3-87 3-57 3-41 82 10-88 7-87 5-15 3-92 3-00 2-75 2-62 62 10-00 7-50 4-69 3-47 2-62 2-40 2-29 42 10-32 7-11 4-22 3-04 2-30 2-10 2-02 22 10-03 0-71 3-74 2-65 2-01 1-85 1-79 22 12-24 9-405 6-79 5-31 4-14 3-42 3-06 350 122 L _ VIII 8-50 6-78 5-35 4-84 4-64 82 7-58 5-74 4-32 3-92 3-68 02 , 7- 10 5-23 3-84 3-40 3-25 42' 6-60 4-71 340 3-01 2-89 22 0-10 4-20 2-98 2-07 2-56 22 9-24 7-23 3-64 4-05 4-16 400 122 11-30 8-93 7-05 6-28 5-92 82 10-37 7-87 5-92 5-15 4-82 62 9-88 7-30 5-35 4-62 4-32 42 9-38 6-73 4-82 4-13 3-86 22 8-88 6-16 4-31 3-70 3-46 22 ' 12-08 9-44 7-36 6-08 5-44 Sags under basic loading conditions shown in italics. CONDUCTORS, SAG AND STRESS CALCULATIONS 25 TABLE III. Hard Drawn Copper Conductors. Tensions in Still Air for Erection Purposes. High Voltage Lines (f -m. Ice). Span in Feet. Temp. Faht. 1P2. 193. 3/-147. 3/-1S. 7/-136. 7/-160. 7/-193. Sq. ins. 02061 02926 05 075 10 15 20 Ib. Ib. Ib. Ib. Ib. Ib. Ib. 150 122 99 200 473 795 1220 1820 2460 82 134 308 722 1 190 1780 2700 3590 62 163 380 865 1405 2 120 3150 4190 42 203 462 1020 1655 2440 3 630 4800 22 256 550 1 170 1920 2740 4180 5510 22 633 874 1 457 2 125 2935 4265 5635 200 122 86 168 420 755 1200 1840 2540 82 99 214 589 1070 1700 2580 3590 62 107 249 704 1270 1980 3040 4140 42 117 294 833 1485 2290 3460 4730 22 131 350 980 1705 2600 3960 5360 22 633 874 1457 212$ 2035 4265 5635 250 122 81 155 389 715 1 170 1850 2570 82 87 177 491 945 1580 2380 3490 62 91 191 564 ] 095 1840 2950 4000 42 95 210 655 1280 2120 3340 4580 22 100 233 761 1475 2620 3770 5 180 22 633 874 1457 2 125 2935 4265 5635 300 122 78 148 372 695 1 160 1870 2650 82 82 161 437 860 1490 2430 3 450 62 84 169 480 975 1710 2790 3950 42 86 178 533 1 110 1950 3190 4470 22 89 189 600 1275 2230 3610 5050 22 633 874 1457 2125 2935 4265 5635 350 122 360 680 1 140 1880 2650 82 403 800 1415 2320 3340 62 431 880 1 590 2 680 3780 42 463 975 1800 3020 4250 22 500 1090 2 050 3410 4 800 22 1457 2 125 2 935 4265 5635 400 122 355 670 1 135 1 890 2720 82 . 386 760 1 350 2310 3 340 62 . 406 820 1490 2 580 3720 42 428 890 1 660 2 880 4160 22 f, 451 , 975 1 850 3220 4 650 22 , , 1457 2 325 2935 4265 5 635 Maximum working tensions allowed by E.G. Regulations shown in italics. OVERHEAD POWER LINES This may be solved by slide rule or graphically as indicated in Fig. 12- In either case a table of cubes of numbers will be found useful. Having obtained the sag in this way, the tension can. easily be found since TD is constant and equal to we have -2 X 25Q 2 2- 765 X 8 8 = 565 Ibs. For 3/-147 at 62 F. TABLE IV. Hard Drawn Copper Conductors. Sags in Still Air for Erection Purposes. Low Voltage Lines (f\-in. Ice). Span in Peek Temp. Faht. 136. 162. 193. 3/-147. 3/-18. 7/-136. 7/-166. 7/-193. Set. ins. 01453 02061 02926 *05 075 10 15 20 feet. feet. feet. fpet. feet. feet. feet. feet. 100 122 044 0-41 0-39 0-41 0-43 0-39 0-41 042 82 0-28 0-26 0-26 0-27 0-28 0-26 0-27 0-27 62 0-24 0-22 0-22 0-23 0-23 0-22 0-23 0-23 42 0-21 0-19 0-19 0-20 0-20 0-19 0-20 0-20 22 0-18 0-17 0-17 0-18 0-18 0-17 0-1S 0-18 22 1-00 0-77 0-615 0-48 0-395 0-32 0-28 0-20 150 122 1-37 .1-07 0-97 0-94 0-95 0-86 0-89 0*90 82 0-88 0-70 0-65 0-64 0-64 ' 0-59 0-60 0-62 62 0-72 0-58 0-55 0-54 0-54 0-51 0-52 0-53 42 0-60 0-50 0-17 0-47 0-47 0-44 0-45 0-46 22 0-51 0-44 0-41 0-42 0-42 0-39 0-40 041 22 2-25 1-73 1-385 1-08 0-89 0-72 063 0-585 200 122 3-00 2-23 1-88 1-71 1-66 1-49 1-50 1-54 82 2-28 1-57 1-31 1-21 M7 1-06 1-07 1-10 62 1-93 1-32 Ml 1-03 1-00 0-92 0-92 0-95 42 1-61 Ml 0-95 0-89 0-87 0-80 0-81 0-83 22 1-34 0-95 0-82 0-78 0-77 0-71 0-71 0-73 22 4-00 3-08 2-46 1-92 1-58 1-28 1-12 1-04 250 122 5-21 3-87 3-14 2-73 2-58 2-28 2-27 2-31 82 442 3-03 2-34 2-02 1-90 1-69 1-67 1-71 62 4-03 2-62 2-00 1-74 1-64 1-46 1-46 148 42 3-61 2-24 3-72 1-61 1-43 1-29 1-28 1-30 22 3-18 1-92 1-48 1-32 1-26 1-14 3-14 1-16 22 6-25 4-81 3-84 3-00 2-47 2-00 1-75 1-625 300 122 7-79 5-95 4-77 4-08 3-69 3-23 3-18 3-19 82 7-04 5-02 3-82 3-18 2-83 2-46 2-42 2-45 62 6-ttt 4-56 3-36 2-78 2-47 2-16 2-12 2-15 42 6-20 4-08 2-93 2-43 2-16 1-90 1-86 1-89 22 5-75 3-61 2-55 2-13 1-91 1-70 1-67 1-69 22 0-00 6-93 5-54 4-32 3-56 2-88 2-52 2-31 Sags under basic loading conditions shown in italics. CONDUCTORS, SAG AND STEESS CALCULATIONS 27 It will be realised from the above example that the calculation of sag and tension is no light task to be undertaken on the spur of the moment. Hence the necessity for working tables. Tables II. and III. give values of sag and tension for High Voltage lines (f -in. ice) and Tables IV. and V. for Low Voltage lines (iV-in. ice). Charts for erection purposes can be prepared from these tables. Curves for 3/-147 (-05 sq. in.) are plotted in Figs. 13 and 14. TABLE V. Hard Drawn Copper Conductors. Tensions in Still Air for Erection Purposes. Low Voltage Lines (f^-in. Ice). Span in Feet. T<nnp. JFaht. 336. 162. 193. 3/-147. 3/-18. 7/-136. 7/-166. 7/-193. Sq. ins. OH53 02 061 02926 05 075 10 15 20 Ib. Ib. Ib. Ib. Ib. Ib. Ib. Ib. 100 122 160 245 360 610" 870 1280 1810 2390 82 250 385 540 925 1 350 1920 2800 3600 62 290 450 640 1090 1600 2220 3230 4290 12 335 525 740 1250 1850 2600 3720 4960 22 390 600 840 1390 2060 2900 4220 5580 22 456 633 874 1457 2125 2935 4265 5635 150 122 115 208 327 600 890 1300 1890 2510 82 179 318 488 880 1320 1900 2790 3630 62 219 385 578 1040 1550 2200 3210 4260 42 263 445 675 1200 1790 2550 3700 4900 . 22 309 507 775 1340 2010 2870 4180 5500 22 456 633 874 1457 2125 2935 4265 5635 200 122 93 178 300 585 900 1340 1980 2610 82 123 253 430 825 1280 1880 2780 3660 62 145 300 508 970 1500 2170 3200 4240 42 174 358 593 1 125 1720 2490 3670 4850 22 209 418 688 1285 1950 2810 4130 5460 22 456 633 874 1457 2125 2935 4265 '5635 250 122 84 160 28a 570 910 1370 2040 2720 82 99 205 376 775 1230 1850 2770 3670 62 108 237 440 900 14-30 2130 3170 4220 42 121 277 511 1 035 1640 2420 3 640 4820 22 137 323 595 I 185 1870 2740 4060 5400 22 456 633 874 1457 2125 2935 4205 5635 300 122 81 150 266 550 915 1390 2100 2830 82 90 178 332 705 1 190 1 820 2760 3690 62 95 196 378 810 1 370 2070 3150 4200 42 102 219 432 925 1 560 2 350 3 610 4780 22 no 248 497 1 055 1 760 2 040 4 000 5 350 22 456 633 874 1467 2125 2935 4265 5635 Maximum working tensions allowed by E.G. Regulations shown in italics. 28 OVERHEAD POWER LINES It will be noted that on the shorter spans the variation of tension with temperature is considerable. The undesirability of using spans of unequal length will be at once clear. If contiguous spans differ very much in length, changes of temperature will cause longitudinal pulls which may result in the conductors slipping at the binders. Variations in span length should not exceed 10 % of the average. The binders will stand up to this difference and the poles are flexible. In cases where abnormally short or long spans are unavoidable, tensioning insulators and longitudinal stays must be used. The remarks in this paragraph do not apply to lines using suspension insulators. Sag at 122 F. in Still Air (see Figs. 15 and 16). These curves (plotted from Tables II. and IV.) are required for determin- ing the height of pole required. The Fig. 15 curves also appear in the first quadrant of Chart (Fig. 51, page 101). It may be worth noting that for conductors up to ! sq. in.' on H.Y. lines and up to -05 sq. in. on L.V. lines the oblique sag under basic loading conditions is greater than the vertical sag in still air (at 122 F. Sag at 62 F. in Still Air (see Figs. 17 and 18). These curves (also plotted from Tables II. and IV.) are required when determining the horizontal spacing between conductors, which will shortly be considered. Sag and Tension under various other Loading Con- ditions. Although the sag and tension tables given are sufficient for practical purposes, the following calculations will be found interesting and instructive. Vertical Sag at 22 F. with f inch Ice only (no wind). Let D m) S m and W^ *efer to basic loading conditions and D i: Si and W i to ice loading only ; W L z W T2 then D m = 4^~ and 7), = ^-, W m T f WJS< Whence & = S m . D<W m ' The Elastic Contraction due to removal of wind 7 Q l ud U voltage lines ! l 5 O Mr, B'lore 30 OVERHEAD POWER LINES 400 350 300 250 200 150 100 34-56 Sag, in Feet 300 FIGS. 15, 16. Sags of copper conductors at 122 F. IDUCTORS, SAG AND STRESS CALCULATIONS 31 2345 Sag, in Feet of copper conductors at 62 P. 32 OVERHEAD POWER LINES This is equal to the change of length of wire in span Equating one to the other, and putting // for I, we get ' ra 87lf V D,-W i '' in Substituting known values for -05 sq. in. copper conductor and. 251 feet span we get 4 7 p 3 _ D a 3 X 29 200 X 250 2 / 4-72 X -521^ 8x18x10 \ A~X '88 /' Whence JD, 3 + 15-7D,- 106-3 = and J3 4 = 3-66 feet. 2 - 521x25Q2 NOW 8D t 8x 3-66 " HP = 22 200 Ib. / sq. in. Vertical Sag at 22 F. in Still Air Without Ice. As above, if D , j$ and W refer to these conditions we have ns ni_ 3 WVi AJF .^02 n 2 3 X 29 200 x 250V, 4-72 X -2 i.e. a-M -- /y = __-_-___ M a x la x 10 \ D Q x -i Whence D S -|- 15-7D ? 40-8 == and D = 2-05 feet, which agrees with, the value given in Table II. which was prepared from equation on. page 22. Vertical Sag at 32 F. with f-inch Ice only (no wind). In this case we have THERMAL EXPANSION = Z/C(32 22) = IQIK. Starting with the ice-loaded conductor at 22 P. in still air, w is constant, therefore Substituting for 8 32 and putting L for I we get ' _ Si JK D 'aa CONDUCTORS, SAG AND STRESS CALCULATIONS 33 Inserting known values n 2 O/VM, 30 X 9-222 X 10~ 6 X250 2 3 X 22 200 x 250V _ 3-66 8X18 X 10~ G ( ~ D __' 13.4 = 2-16 - 28-9 "Whence 7) 32 3 + 13-31-Z> 32 - 105-6 = and D 32 ^=3-8 feet. Oblique Sag at 32 F. with f-inch Ice and 8 Ib. wind. Starting from basic loading conditions THERMAL EXPANSION = Z#(32 22) = lOZ/i ELASTIC CONTRACTION = (S m S 32 ) ~ W is constant .-. S m l) m S.^D' SZ . Substituting for $ 32 and putting L for I we have Inserting known values 30 X 9-222 x 10- X 250 2 3 X 29 200 X 250V 4-72 8X 18 X 10 179-5 D 3 o 2 - 22-3 = 216 - 38 + -^r -L'sa Whence D 32 3 + 13-54D 3a - 179-5 = and D 32 = 4-85 feet. 250 V. 4-72\ -^~( L ~irJ'- The maximum hypothetical horizontal displacement of the con- ductor (4-85 x = 3-92 feet) occurs with this loading. '88 Oblique Sag at 62 F, with 15 Ib. Wind. The wind pressure of 8 Ib. per square foot corresponds with a wind velocity of 50 miles per hour, which it is considered sufficient to allow simul- taneously with an ice loading. It is not the highest value likely to be experienced ; in fact, velocities exceeding 100 miles per hour have been recorded and 70 miles per hour is quite common at higher 3 34 OVERHEAD POWER LINES temperatures. This is of no importance so far as the strength of the wire is concerned but it may cause trouble due to the wires swinging together (see p. 42). We will therefore consider the state of affairs with 15 Ib. wind pressure at 62 F. Let the subscript " w " refer to values for the wind loaded wire and " o " to the wire in still air. , QJ. o from which T w = T Q . ^ and ^ = "" Q-Lsw Increase of Stress due to wind loading W T) "' From Table I., weight of conductor per foot run = -2 Ib. and diameter = -317 inch: IK Wind pressure = -317 X ^ = -396 Ib. / ft. run and W u = V'396 2 + -2 2 = -443 Ib. / ft. run. From Tables II. and III., D = 2- 765 feet, and T = 505 Ib. 565 * ...S = s _= 11 300 Ib. / sq. inch. Now, substituting known values in equation we get D 2 _ 9. 7652 _ 3 X 11 300 X 250V -443 X 2- 765 w " ~~ Ijghence D^s + 7-12^ _ 90 = and , /), = 3-95 feet. .. The triangle of forces acting on the wire is shown in Fig. 19. Therefore the horizontal displacement of the wire will be .OQtf 3 ' 95x - = 3-54 feet. J96 CONDUCTORS, SAG AND STRESS CALCULATIONS 35 The above results are shown diagrammatically in Fig. 20. The angles of deflection for 7/-122 aluminium and 7/-132 steel-cored aluminium, which are electrically equivalent to 3/-147 copper, are shown also for comparison. The figures shown are for a span length of 190 feet in the case of 7/-122 aluminium and of 310 feet in the case of 7/-132 steel-cored aluminium, these span lengths requiring about the same sag at 122 F. in still air as 3/-147 copper on a 250 feet span. (See Chap. X., p. 183, and Tables XVIII. FlG ' 19< and XIX.) This figure will be found useful when considering the horizontal spacing to be allowed between conductors (see p. 42). 3-68' 15 IBs. Wind 62"F.15lbs.Wind 15lba.Wind ^a Ice, 8lbs. Wind 62 F Still Air ( 22 F 3 4lce, 8 Ibs, Wind N % X 22F. 9& Ice. fl Ibs.Wind - !32"F. %' Ice, 8 Ibs. Wind 3-92' >J 22 F.9& Ice, Still Air 32"F.%"lce,StiUAir 12ZF. Still Air ^ 4 . 02 - @4 . 15 . 0) Copper 3/- 147 250 ft. Span Aluminium 7/-122 190 Steel Cored Aluminium 7/-132 310 FIG. 20. Diagram to illustrate movement of conductors under various loading conditions. (H.V. loading.) Notes on the above Calculations. No great precision ota be claimed for the figures given in the Tables II. to V. We cannot be certain of the exact values of the modulus of elas- ticity and of the coefficient of linear expansion, and tolerances of 1 % and 2 % are allowed on the conductor diameter and weight respectively. But it must be realised that the lineman cannot in practice avoid 36 OVERHEAD POWER LINES errors of a few inches or so when adjusting the sag and the tempera- ture at the time of erection can only be estimated. Even if a ther- mometer is used, the temperature of the wire in a hot sun may be much greater than the air temperature. The Tables show the minimum sag required by the Regulations, but it is not always necessary or desirable to pull up to the legal limit. There is no point in pulling up to sags less than (say) 1 foot. Frequently in short span work, larger sags are given than are neces- sary from a legal point of view in order to reduce the value of the terminal stresses. Unfortunately, the percentage increase of sag to be allowed on erection in such cases is much greater than the desired percentage decrease in terminal stress under the worst loading conditions. As the Tables cannot therefore be used, the following method of cal- culating sags and. stresses is suggested. Erection Sags and Tensions with Reduced Basic Loading Stress. Having decided upon the maximum stress to be allowed, first find the sag at the critical temperature and then the sag at 22 F. without ice and wind. The sag (and stress) at other temperatures may then be found with sufficient accuracy for practical purposes by assuming a straight lino law of variation of sag (and stress) with temperature. For example, consider a 7/-166 conductor on a low voltage line pulled up to half the maximum tension allowed by the Regulations, i.e. 14 250 Ib. / sq. in. under basic loading conditions (Table I., p. 4). (1) Sag at Critical Temperature. 14250/ -595\ ""^""" ~' Whence B = 54 F. Assume a 200 feet span. From Table III., page 25, the sag under basic loading condi- tions ( T \-in. ice) with 'S m = 28 500 Ib. / sq. in. = 1-12 feel There- fore the sag under the same loading conditions with half the stress will be increased to l-]2 X 2 = 2-24 feet and this will also be the value of the sag at the critical temperature 54 F. (2) Sag at 22 F. without Ice and Wind (see p. 32). r> 2 _ 7) ?, ^rnL' m 8M CONDUCTORS, SAG AND STKESS CALCULATIONS 37 Inserting known values, we have . 12 n 2 __ 3 X 14 250 X 200V, 2-24 X -595\ " Whence We now have and 8 X 18 X 10 6 V 1 D 3 -f 6-88 DO - 16-65 = 0. D = 1-70 feet. 5=22F., D = 1-70 feet, B '= 58 F., D = 2-24. feet. X -95 /' 3-5 3-0 2-5 True Sag for- S m m. 14,250 lbs./sq.in, under Basic Load- Ing Conditions 2-0 CO 7-5 1-0 0-5 Straight Line through Points Sag- 20 40 60 80 100 120 140 Degrees Fahr. Fro. 21. Erection sags for 7/-100 copper, 200 ft. span. Low voltage linos. These points are plotted in Fig. 21 and the straight line drawn through them is seen to be in close agreement with the more exact curve obtained in the manner described on pages 21-23. As a matter of fact, this shorter method of treatment gives fairly good results in most cases, but it is not advisable to use it when working at stresses near the legal maximum. Solid Versus Stranded Conductors. For sections above 0-1 sq. in., stranded conductors must be used as solid conductors 38 OVERHEAD POWER LINES become too unwieldy to handle, but between about -03 and -075 sq. in. practice differs according to the experience of the engineer; Stranded conductors are undoubtedly easier to handle arid less liable to serious damage due to want of skill or carelessness. For the same cross-section, the stranded conductor is a little more expensive than the solid ; the stranded has the larger diameter and therefore a larger wind load, but on the other hand the safe working load of the stranded conductor is higher, and it will be found that the sag to be given to a conductor of given cross-section is sensibly the same whether the conductor is stranded or not. It may be noted that it is not usual to carry the stranding so far with H.D. copper conductors as with the annealed copper conductors used in cables. For example, a standard -15 sq. in. conductor has 7 strands of -166 inch diameter in overhead line work and 37 strands of -072 inch diameter in cable work. Notes on Sag Adjustment During Erection. It will be found that when the conductor is erected, the final tension to be allowed is relatively low and may be insufficient to smooth out any slight kinks there may be and to take the initial stretch out of the material. It is usual, therefore, to pull up the conductor to GO % of its breaking load (i.e. about 20 % greater than the safe working load values given in Column 8, Table I., p. 4) and to maintain this load for a few minutes. It may then be assumed that for practical pur- poses, the stress will be proportional to the strain over the normal working range which is, of course, assumed throughout in the above calculations. This applies to both solid and stranded con- ductors. If it is impracticable to apply such largo loads as the above implies with the larger conductors, then as great a load as possible should be applied and maintained for a couple of clays or so. Having " killed " the wire in this manner, the final sag adjustment can be made as follows : (1) Points of Support at Same Level (Fig. 22 (a)). Mark oil on the supports the distances AM and BN each equal to the appropriate sag from Tables II. or IV. and then .adjust the conductor until it just appears in the line of sight between M and N. If the sag is small it is desirable to check the result with a spring dynamometer, using the figures in Tables III. and V. ' Another method of checking the sag is referred to on page 42. CONDUCTORS, SAG AND STRESS CALCULATIONS 39 (2) Points of Support at Different Levels (Fig. 22 (6)). The treatment in this chapter is only strictly correct when the supports are on the same level. When the spans are long and the differences of level large, the design requires special consideration, which is outside the scope of this book, but for short span con- struction on moderate slopes no difficulties are likely to arise from using the tables of sags and stresses given and proceeding as above. The length of span must be taken as the horizontal distance between the supports (L) in all cases. On a slope of 1 in 3, however, Fro. 22(). FlG. 22 (b). the length of slope I/(Mg. 22 (6)) exceeds L* by about 6 % only. It is to be noted also that if the ground surface is parallel to AB and MN, then the vertical distances MP and QH are about 6 % greater than the minimum ground clearance GJ. Therefore, for a given horizontal distance between the supports the poles will have to be somewhat longer on an incline than on level ground. There may be an upward pull at the lower support A, but in ordinary circumstances this is of no importance when pin insulators are used. CHAPTER III. CONDUCTOR ARRANGEMENT, CLEARANCES AND SPACING. We have to consider (1) Clearance of line conductors from ground. (2) Clearance of earth, wire and auxiliary conductors from ground. (3) Clearance of line conductors from pole and pole ironwork. (4) Spacing between line conductors. The earth wire may be considered as a line conductor as far as spacing is con- cerned. The clearances for (1) and (2) are fixed by the E.C. Regulations (see Appendix I.) and are as follows. The figures are the minimum allowed by the regulations at 122 K, the assumed maximum slimmer sun temperature in this country : (1) Clearance of Line Conductors from Ground. (<?.) HIGH VOLTAGE LINES. In all situations at all voltages up to G6 000 20 feet. (b) Low AND MEDIUM VOLTAGE LINES. (i) Public road crossings . . . 19 (ii) Situations inaccessible to vehicular traffic 15 (iii) All other positions . . . 17 (2) Clearance of Earth Wire and Auxiliary Con- ductors from Ground. (a) HIGH VOLTAGE LINES. (i) When erected across a public road or canal or across a railway . . . .20 feet, (ii) Situations inaccessible to vehicular traffic 15 ,, (iii) All other positions . . . . 17 (b) Low AND MEDIUM VOLTAGE LINES. (i) Public road crossings . . . 19 (ii) Situations inaccessible to vehicular traffic 15 (iii) All other positions . . . 17 CONDUCTOR ARRANGEMENT 41 (3) Clearance of Conductor from Pole and Pole Iron- work. It will be noted in Kg. 38, page 56, that the dry spark over distance on a typical 11 000 volt insulator is about*? inches. This may be taken as a rough guide to the working clearance which should be allowed between conductors and metal cross-arms,. earthing brackets, etc. The following minimum values are suggested : Up to 660 volts 6600 11 000 22 000 33 000 4 inches. 6 9 10 12- Bird Trouble. A good deal of trouble is sometimes experienced due to birds settling on the wires or pole ironwork and causing short circuits or earths, mainly the latter. The result is generally disas- OR PORCELAIN. FIG. 23. Fia. 24. trous for the bird but unfortunately its electrocution frequently causes an interruption of supply due to the operation of the leakage protective device. This, of course, only occurs on high voltage systems, and the higher the voltage the greater the nuisance. To obviate the trouble we may either, (a) ALLOW LARGER CLEARANCES, as for example by using longer insulator pins than would otherwise be necessary. (b) DISCOURAGE BIRDS PROM SETTLING. The designs shown in Figs. 30, 31 and 32, pages 46-48, appear to be quite satisfactory. Birds are seldom found to settle on the slanting surfaces provided. (c) PROVIDE INSULATED PERCHES OR " BIRD GUARDS." Two types are illustrated in Figs. 23 and 24. In this connection it is to be noted that bird guards are not necessary with oak arms, if the earth connections to the insulator pins are fixed under the arms. 42 OVERHEAD POWER LINES A rubber-wax compound known as Pernax, which is a tough, flexible and durable insulating material of high dielectric strength, obtainable in sheet and tube form from the Groydon Cable Works, can also be recommended for wrapping round arms, conductors and stay wires. (4) Spacing Between Conductors. It will be obvious that the closer the conductors are together the better from a mechanical point of view, since shorter poles and smaller and lighter pole fittings can be used. Up to (say) 33 000 the working voltage has little bearing on the matter. The HORIZONTAL SPACING will first be considered. This is decided mainly from the possibility of the conductors blowing to- gether in strong winds. Smaller conductors should, therefore, have relatively larger horizontal spacing, not only because the sags are larger, but also because of the greater ratio of wind loading to weight, which results in a greater displacement from the vertical for a given wind pressure. Table XIV., page 106, illustrates this point. If a \vire hanging freely in a parabolic curve is displaced from the vertical and then released, it will swing regularly, and, considered as a compound pendulum, it can be shown that the relation between the sag in feet (D) and the number of half swings per minute (N} is given by the equation D 7^2 This relationship is plotted for sags up to 1 feet, in Fig. 25, which will be found useful when checking sags during erection. If, therefore, all the conductors are erected with precisely the same sag they should swing together synchronously, and there should be nothing to fear from contacts, but unfortunately exact equality of sag is difficult to effect and maintain in practice, and consequently the various conductors may have different periods of swing. Actually, of course, in a gusty wind the movements are very erratic, particu- larly of the smaller and lighter conductors. From a theoretical point of view, to render it physically impossible for two conductors in the same horizontal plane to touch one another, it would be necessary to allow a spacing equal to twice the maximum horizontal displace- ment of each conductor due to the highest wind pressure likely to be, experienced. This assumes the conductors to swing 180 degrees out of phase, but a little thought will show that such a contingency is very remote, and it is found practically that a spacing about equal CONDUCTOR ARRANGEMENT to the maximum horizontal displacement gives a good facto: safety . Reference to Table XIV. (p. 106) and Fig. 20 (p. 35) will si that there is some justification for the following practical rules H.V. lines. (Somewhat smaller spacings, say, 20 % less, will usu suffice for L.V. lines, with a minimum of 1 foot) : HORIZONTAL SPACING (that is, the distance between conduc" when fixed in the same horizontal plane, as in Figs. 26 and 27) :- Copper. Allow a spacing equal to sag in still air at 62 F. Aluminium, ,, ,, 1|- times ,, ,, ,, Steel Cored Aluminium. Allow a spacing equal to 1|- times in still air at 62 F. N? of Half Swings pen Minute (N) _. ^9 -fc. O Co O N "2. Q Q g c \ \ \ D= 14.6C ^ ^-^ ~~-^^ Z 4 6 8 10 12 Sag, In Feet (D) Fio. 25. Measurement of sags by swings. VERTICAL SPACING. In this case there is no danger of the TA swinging together if equally loaded, but we have to consider possibility of an upper conductor becoming more heavily IOE than a lower one due to unequal quantities of ice or flocks of b' Moreover, a numbe'r*of birds settling on a lower conductor at s distance from centre of span may cause the conductor to be li up in the other half of the span sufficient to make contact with upper conductor. It is generally inadvisable to fix high vol 44 OVERHEAD POWER LINES conductors above one another exactly in the same vertical plane for these reasons (Fig. 28). Logically, the vertical spacing should bear some relationship to the ratio of ice load to the weight of the wire alone, but the practical rule which is usually worked to is : VEETICAL SPACING (Copper Conductors). Allow 1 foot per 100 feet length of span (with a minimum of 1 foot). This spacing gives a reasonable factor of safety for conductors up to 3/-18 (-075 sq. in.), but is on the generous side for the larger copper conductors. Perhaps it should be made clear that by " vertical spacing " is meant the vertical distance between horizontal planes through the conductors. on the same side of the pole. Somewhat larger vertical spacings should be allowed with the smaller aluminium and steel-cored aluminium conductors. It is to be remarked, however, that a large number of lines, both in this country and abroad, appear to be giving satisfactory service with spacings much less than these rules demand. On, long spans, of the order of 600 to 900 feet, it is observed that the wires do swing synchronously, the deflection in strong winds being several times as great as the spacing between conductors. It will be noted in Fig. 25 that the rate of change of N with D falls rapidly with the larger sags, and, further, the small differences in sag inevitable in erection are comparatively insignificant, expressed as percentages. More- over, comparatively large conductors are generally used on these long spans. Arrangement of Conductors. HIGH VOLTAGE LINES. Figs. 26 to 32 show examples of con- ductor arrangement on single circuit H.V. distribution lines, with earth wire but without auxiliary conductors for protective gear or telephones. The clearances shown in the figures are suitable for 3/-147 conductor on 250 feet spans. The mechanical strength of the pole fittings will be considered later. It will be found generally that a horizontal arrangement of con- ductors allows a shorter polo to be used and this may be of importance in some cases, unless the vertical arrangement is indicated by other considerations. An equilateral triangular arrangement (Figs. 20, 27 and 32) is best from a purely electrical point of view and is therefore desirable, if nothing is lost thereby ; but this is not a ruling factor in CONDUCTOR ARRANGEMENT '"! 9 j 2 f. Scale. Rivcftcd. Chonnal.4xZ' 9" .1 --- _ 2-9" "^cS 1 Fio. 20. rfht. Wire. / ChoBnel.SI'/i i___ . 2'-9' I 2-6" Fia. 27. Earfh Wire. FIG. 28. Fio. 29. Simple typos of H.V. polo fittings. OVERHEAD POWER LINES FIG. 30. Type of H.V. pole fittings. CONDUCTOR AURA NGEMENT channel Scale "to Eastern fe/y N3/V C/famnce f/o/fs FIG. 31, Type of H.V. pole fittings. 48 OVERHEAD POWER LINES T , i , I ,___?,,. onn , 's standard design.) 20 000 volts, double insulators and wire guards. CONDUCTOR ARRANGEMENT 49 distribution work, and if the nearest standard pole is a little on the short side, the vertical distance between the conductors in these three designs may safely be reduced to 30 inches. Figs. 30 and 31 are examples of the " tilted " triangular arrange- ment which is favoured by some engineers, there being no two conductors in the same plane, horizontally or vertically. It can easily be verified by trial that, for the same spacing between con- ductors,' the factor of safety against contacts is greater than with the arrangement of Figs. 26, 27 and 32, in which the two lower conductors axe in the same horizontal plane. i\ Figs. 26 and 32 show double insulators which are specified by the \\ Electricity Commissioners in the neighbourhood of roads. Although the conductor clearance from the ground must be at least 20 feet in all cases, a clearance of 15 feet only is required for the earth wire across country, and it may, therefore, be two or three feet lower than shown in the figures. Advantage may be taken of this to ease up the load on the pole, but the higher the earth wire is fixed, the more effective it is as regards atmospheric effects. Low VOLTAGE LINES. Some examples of L.V. conductor arrangement are given in Figs. 33 to 36. L.V. distribution is invariably short span work on account of service connection which must be taken of! at the poles, and for which reason it is frequently more convenient to arrange the con- ductors in a vertical plane (Figs. 35 and 36) in spite of the somewhat longer poles thereby necessitated.' The use of poles for street light- ing also has a bearing on the question of span length. Owing to the relatively short spans and the less onerous hypothetical loading conditions it is not usually necessary or desirable to pull up the conductors to the limit of tension allowed by the regulations, as the increased difficulty and cost of dealing with the stresses at angles and terminals is far greater than any saving which might be effected in the cost of supporting poles. It is, of course, necessary, mainly for aesthetic reasons, to allow the same sags on all sizes of conductors on the same poles, but in cases where this point arises, the span length seldom exceeds 150 feet. Fig. 3.3 shows an arrangement suitable for a 4-wire L.V. feeder (without branches), with a " split " neutral, which was required by the old (1923) B.C. Regulations. The new (1928) Regulations permit a single wire neutral to l?e used with this design, provided the wire 4 OVKKIIKA1) POWER LINKS Eta. 34. Typos of L.V, polo fittingB. CONDUCTOR ARRANGEMENT . 30. Typos of L.V. polo fittings. 52. OVERHEAD POWER LINES is staggered from one side of tlie pole to the other, but this appears to present practical difficulties. Pig. 34 shows 3-phase wires, 1 switch wire, and a " split " neutral, also erected to comply with the old Regulations. Fig. 35 shows a vertical arrangement which now complies with the Regulations, the single wire neutral being considered sufficient as a guard wire when it is directly below the phase conductors. Kg. 36 shows a vertical arrangement with a " V " guard, re- quired by the old Regulations, but now no longer necessary. As the insulation of the shackle (Kg. 35) is inferior to that of the shed insulator (Fig. 36) it is better to use the latter type as far as possible in straight rims and for small angles, and reserve the former for terminals and considerable angles. H.V. and L.V. Lines on Same Poles. The idea of using the same poles for both H.V. and L.V. lines is, of course, by no means new. It has been common practice on the Continent and in America for many years, but has hitherto not been encouraged in this country. Fig. 37 shows a combined pole which has recently been approved by the Electricity Commissioners and which presents many points of interest. The Shropshire, Staffordshire and Worcestershire Electric Power Company have gone very carefully into the question of standardisa- tion and the design shown is typical of their standard practice in rural distribution. All drilling and slotting of poles is done before despatch to site. Every pole is slotted to take an 8 in. X 4 in. X 4 ft. foundation block 1 foot 6 inches below ground. This enables a foundation block to be readily fitted if, on excavation, the engineer decides that the nature of the ground is such as to make it necessary. The buried depth of 5 feet G inches should be ample for most common types of soil, but the 1928 E.G. Regulations are really more onerous than the old ones concerning pole foundations. The vertical spacings between conductors is rather less than is recommended elsewhere in this book, but the values allowed have been found to give satisfactory service in the area of supply con- cerned. The minimum clearance between H.V. and L.V. lines is 19 inches. The pole brackets will be seen, to be of very simple design. Ordinary standard insulator pins are used with distance pieces of iron tube between the strap brackets. One L.V. phase conductor only is shown, but the poles are of sufficient diameter and length to accommodate CONDUCTOR ARRANGEMENT 3-phase conductors, when required. The L.V. phase conductors are insulated with P.B.J. insulation (see Specification, Appendix IV., p. 231). The short pieces of straight iron strap marked " a " on the L.V. Carfbcd Ctonhnl Conductor. 05'= fcD97 H.D. porcclairj lasulaiors- Drowc porcukiit2 Irasalatbrs. Ebrl'hiGg Bow Hxterads OutwardsH'. 05""- 7-097. R5.J. Insulated conductor. wre bare copper. LEMGTH or POLES. 54 1 . Dia: AT 3' FROM MAXIMUM SPAN. 150'. MINIMUM CLEARANCE BETWEEN H.V. <&LV. LINES. H9". i DEPTH In GROUPID. 5'-6". PIG. 37. H.V. [3 300 volts] and L.V. [230 volts to earth] on same pole [Slirop- 4 shire, Staffordshire and Worcestershire Electric Power Company]. fittings are drilled so that when turned outwards from the pole they are readily available for service connections by the addition of porcelain pulley insulators. No continuous overhead earth wire is used, the H.V. ironwork 54 OVERHEAD POWER LINES being connected to an earth plate at every pole. The earth i nccting wire is covered with creosoted wood casing for a distanc 9 feet /Tom the ground. Earthing bows are fitted on all pole The L.V. ironwork is connected to the earthed neutral conclur This is sound practice as it puts the ironwork definitely at ej potential (or .nearly so) and although contrary to the E.G. Reg lions aw they stand at pmsent, it has been specially approved in instance. 11; may be pointed out that it would bo undesirable to com the L.V. and H.V. ironwork to the same earth, a,s the earth resists juuy not bo low enough to obviate dangerous voltages in the I wystein in case of insulator breakdowns. CHAPTER IV. INSULATORS. THE material for .overhead line insulators must possess a high diele trie strength and insulation resistance, and the insulator should ' so shaped as to minimise concentrations of dielectric stress due surges which might puncture the material and so render it uns< viceable. The shape is, of course, a matter for the designer , and the operating engineer can do is to specify a high ratio of puncti voltage to flash-over voltage. A flash-over will probably cause interruption of supply by operating the protective gear, but the sup] can be restored immediately without the delay which is inevita" if one or more punctured insulators have to be located. In addition to the electrical properties mentioned above, 1 insulator must, of course, have sufficient mechanical strength support the conductor under all weather conditions. The material which, in this country, is considered most nea to satisfy all the required conditions is Porcelain, the mamifact of which has reached a very high standard. The porcelain must absolutely vitreous throughout to render it non-hygroscopic. . surface is glazed, not to improve the insulation, but to render diffic the deposit of dirt which increases surface leakage, and to facilii the washing off by rain of whatever dirt does settle. The smo glazed surface also reduces the wear of the conductor by abras: The body of the porcelain should be ivory white but the glazing i be of any colour, brown being considered the best, as the insula then form less conspicuous targets for small boys. Porcelain insulators are standardised for H.V. lines (B.S. S| fication 1371922). Pin Insulators, High Voltage. ELECTRICAL DESIGN. For voltages up to 33 000, pin type sulators are invariably used. 56 OVERHEAD POWER LINES The various ways in which such an insulator may fail electrically will be clear from Kg. 38 and the table of particulars which follows : Particulars of Typical Standard 1 1 000 Volt Pin Insulator. Inches. British Standard Test Voltages. (1) Puncture thickness .... 75 108 000 (2) Arcing distance (dry) A -f B + + D. 5-0 62000 f (3) Arcing distance (wet) a -|- b -|- c . 2-25 39000 (4) Leakage distance .... 7-0 If the insulator is correctly proportioned with regard to wet and dry arcing distances, the leakage will be negligible. MECHANICAL DESIGN. Two strengths are standardised, viz. : 400 Ib. and 800 Ib. As manu- factured, the insulator it- self is generally suitable for 800 Ib. and the strength is determined by the size of the pin. He- ference to Table I. shows that the 400 Ib. design is suitable for supporting poles up to a span length of about 400 feet. The 800 Ib. insulator comes in for longer spans and also- for angles, where the lateral loading includes an appreciable component of the longitudinal tension in the conductors as well as the lateral wind pressure. Pin insulators are not Fid. 38. 11 000 volts porcelain insulator, Leakage distance = 7 ins. Dry spark over distance A + B + C-j-D = 5in Wet spark over distance a + b + c = 2 ins, Puncture thickness = f in. INSULATORS 57 often used at terminal poles, but they are quite suitable for some of the smaller conductors. It is important that there should be no appreciable deflection of the insulator pin under load, to avoid possible fracture of the por- celain. The B.S. Specification above referred to lays down a V of $ of 2|- based on the yield point. V/ra V/ORKIN& 33 K\/ PAIN TELOT 84- DRY 3PARK OVEB TEST 125 KV PUNCTURE TEST 220 - WORKING LOAD 400 Iba WEIGHTS .- INSULATOR 15k w. R.T D.30.T 62" P.T. 108 WL. 400 Ibs. Wra a%3 2fc Ifo Wra PiG. 39. Typical high voltage porcelain insulators and pins. Fig. 39 shows a series of B.E.S.A. Standard high voltage por- celain insulators. It is to be observed, however, that trouble has been experienced with these insulators in very exposed positions, particularly near the sea, and it is the practi.ce of some engineers to use an insulator one grade higher than the British Standard rating. 58 OVERHEAD POWER LINES For example, an 11 000 volt Standard insulator would be used on a 6 600 volt system. Tt is understood that B.S.S. 137 will shortly be revised. Loids on Insulators at Angles. First consider a straight line pole (Fig. 40 (a)). I -fir Fia. 40 (a). For simplicity, assume the whole of the load on the conductor to be horizontal and equal to P lb. per span.. If the tangent to the conductor at the point of support makes an angle oc with the direction of the line, the lateral load on the insulator P = 2T sin oc lb., T being the tension in the conductor. The longitudinal forces balance. FIG, 40(6). Now suppose the span BG to swing round through 6 degrees (Fig 40(6)), then the resultant horizontal force on the insulator = P 1 / f) =; 2T cos j8 = 2T sin ( oc + - \ -a 2T sin oc cos - -f 2T cos oc sin 2 = P cos + 2T sin - cos oc. 2i 2 INSULATORS 59 In all practical cases the angle oc is in a plane inclined to the horizontal, as explained in Chapter II., but the reasoning still holds if it be remembered that T is actually due to the weight of wire + ice loading, as well as to wind pressure. ' Now tan oc = and is of the order r-^-. Ju LJ 100 .-. tan cc = -04 and cos oc -999. The effect of the angle cc 011 the result is therefore negligible. a If 8 -= 45, cos = -925, and with the smallest permissible copper conductor on a 200 feet span, P = 122 Ib. on high voltage lines and T m 633 Ib. Substituting these values in the above formula we get n (i) Neglecting cos -, P l = 606 Ib. 2i (ii) Including cos -, P : 597 Ib. The neglect of cos therefore introduces an error of 1-5 L 2i *f on the safe side. We may then with sufficient accuracy for practical purposes omit a the factors cos - and cos oc and write : 2 RESULTANT HORIZONTAL LOAD ON INSULATOR Values of 2 sin - are plotted in Fig. 41 . /! As a matter of fact this formula gives pessimistic values not only because of the omission of the two factors referred to above, but also because the wind cannot blow at right angles to both spans simultaneously. In our example, using 3/-14-7 copper conductor on a 250 feet span, P = -710 X 25'0 = 177-5 Ib., and T m = I 4-57 Ib. If is the maximum deviation permissible, we have 60 OVERHEAD POWER LINES For the 400 Ib. pin. 400 = 177-5 4- 2 . 1457 sin --, a from wliich 2 sin. - = -153, and therefore, from Kg. 41, 7.4. 7-2 f-0 0-8 0-6 0-4 0-2 7 20 40 60 80 100 Degree s Q FIG, 41. -For the 800 Z6. pn. 800 = 177-5 4- 2- 1457 sin |, whence 2 sin = -428 autl 6 = 25. a If tlie angle pole is double armed and two 400 Ib. pin insnlato] are used, we may assume that they share the load equally, and ther< fore the same angle (i.e. 25) can be negotiated as with an 800 Ib. pii Table VI. shows the maximum, angles which can be dealt wit by the two standard sizes of pin on the span lengths suggested i Table XIV., page 106. INSULATORS 61 TABLE VI. Maximum Angles for Standard H.V. Pin Insulators. Wind Load, Maximum Length, feet. H.V. Loading P, Ib. Longitudinal Tension, T, Ib. 400 Ib. Pin, degrees. 800 Ib. Pin, degrees. 162 . 200 122 633 25-0 65-0 193 . 250 157 874 16-0 42-0 3/-147 . 280 199 1457 8-0 25-0 3/-18 . 315 239 2 125 4-0 16-0 7/-13G . 335 258 2935 2-5 11-0 7/-160 . 350 291 4265 1-5 7-0 7/-193 . 335 297 5 635 1-0 5-0 It will be seen from the above figures tliat the heavy loads due to the larger conductors make it desirable to avoid small angles and to adhere as far as possible to absolutely straight runs between definitely strengthened angle poles with tensioning insulators. In practice, however, these standard pins are frequently used for larger angles than those given, although calculations show that tensioning insulators should really be used. Immunity from trouble in such cases is undoubtedly due to the fact that the maxinram hypo- thetical loading conditions are rarely experienced. In this connection it may be repeated that it is often desirable to allow somewhat larger sags than the E.G. Regulations demand in order to keep down the values of the longitudinal stresses at angles and terminals. This remark applies particularly to L.V. lines in which, relatively, the spans are short and the conductors large. v Methods of Securing Insulator to Pin. The following methods are commonly employed : (1) Pin screwed directly into the porcelain. (2) Pin screwed into a metal thimble, which is cemented into the insulator. (3) Insulator cemented on to the pin. The first two methods are generally to be preferred as they permit the insulators and pins to be transported and handled separately and of insulators being easily replaced. Moreover, the fitting is done in the factory instead of on the job. But in the first method, the hard unyielding joint is likely to lead to cracking of the porcelain under temperature changes, and the sharp edges of the metal thread are undesirable from an electrical point of view. These disadvan- tages are not serious with small insulators and low voltages and an india-rubber or felt washer on the shoulder of the pin minimises the ( OVERHEAD POWER LINES effects of unequal expansion of the steel pin and the porcelain i lessens the risk of fracture when screwing on. For large insulators at high voltages the second method is variably used in this country (see Fig. 38,, p. 56). The third method, that of cementing the insulator on to - pin, makes a sound job when well done, and it may have to be sorted to abroad. Care must be taken to use a cement that will i act chemically on the pin so as to produce a substance which expai and breaks the porcelain. Sulphur must on no account be used Neat Portland Cement (B.S. Specification 121 920) is the saf material for the purpose. As the insulators reach a very hi temperature in the sun it is fortunate that the coefficient of them expansion of iron, portland cement and porcelain are not very c forent. Iron has a rather larger coefficient than the other two, 1 ifc happens that the cement has the smallest modulus of elastic and strength, under compression which enables it to act as a cush: between the iron and the porcelain.. The cement mixture should be in the ratio of 1 pint of wa to 4- 11). of cement, in which proportion it has a semi-fluid ci fdstoncy. Care must be taken to fix the pin centrally in the hole of the milator and to ensure particularly that there is a layer of ceim between the end of the pin and the bottom of the hole. Tho cement takes at least 48 hours to set, but the insulators c bo removed from the framework after 24 hours, if handled careful PLANTER OF PAULS is sometimes used and appears to give sal factory service, though not nearly so strong as cement. It has 1 advantage of setting more quickly. To prepare the mixture, rn some ordinary carpenter's glue to the consistency usual for wo joints and dissolve it at the rate of >}<$ pint of glue to one gall of wator. Then make a plaster of paris mixture with the consisten of soft putty. Although the setting commences in about 40 minut the insulator must not be moved for 2 hours, nor placed in positi on pole for 24 hours. It is perhaps unnecessary to say that this cementing on shoi not bo done in frosty weather. v/Methods of Securing Wire to Insulator. It will obvious that the side groove must be used at angles. Whether t top groove or the side groove is used in the straight is a matter opinion, but the top is most largely used. INSULATORS 63 Bronie Clip Vertical Bronze Wedges FIG. 42. Side groove clip. It must be remembered that the insulator is standardised for a pull on the side groove, and the bending moment on the pin is some- what greater when the wire is in the top. Moreover it is impos- , ^^s \ Bronze Sheath sible to make such a /^ L^ slit dov ynwands) good job of the bin cl- ing-in when the wire is in the top groove. For heavy conductors the top groove must be used, as the line- man cannot hold the wire in the side groove when making of. It is an advan- tage for the wire to lie in the side groove on the pole side of the insulator as there is then less chance of the wire falling if the insulator is broken. The ideal binder should be strong enough to prevent the wire from slipping to and fro through it with every change in temperature due to inequality of span lengths, but it should allow the wire to slip before the elastic limit of the pin is reached. It should also be as flexible as possible to prevent the setting up of crystallisa- tion in the conductor. The use of ME- CHANICAL CLIPS is sometimes preferred to the usual method of attaching conductors to insulators by means of binding wire. Fig. 42 illustrates a clip for the side groove and Fig. 43 one for Slotted hexagon-head studs with square nuts FIG. 43. Top groove clip. OVERHEAD TOWEJl LINES the top. The side groove clip requires a special tool but makes good job and is certainly a time saver. It is made with ho zontal wedges for use when the insulators have flat upper shec The top groove clip which is of soft copper also makes a good jc saves time and the only tool required is a screwdriver. These rr chanical clips (especially the top groove design) naturally cost good deal more than binding wire, but on the other hand they reqiii less skill and time. The following methods of binding-in and terminating are sit gested. Table VII. gives the lengths of binding wire required f< the various sizes of conductor. The use of side-cutting pliers shou be forbidden. Side Groove. Starting with the middle of the binding wire i point P (Fig. 4-: serve the condut tor for a lengt equal to the dk meter of the nee of the insulator. Take the en< which leads ol from the top o the line wire (cal this end A), pas; it round the necl of the insulatoi and take a rounc turn from above downwards, round the conductor. Then take the other end (B], pass it round the neck of the insu- lator and take a round turn from below upwards round the conductor. Finally, pass both ends round the neck of the insulator again and finish off with a serving on the conductor of about 2 inches oil each side of the insulator, end A winding from below upwards and end B from above downwards. If these instructions are carefully followed the turns of wire round the neck of the insulator will not ride one upon another. Top Groove. Divide the length of binding wire given, in Table VII into two equal paits and lay up together to form a double wire, leaving Z inches of single wire at each end (see Table VII. , last column), FIG. 44. Side groove binding. INSULATORS 65 Starting with the middle of the double binding wire at point P (Fig. 45) serve the conductor for a distance equal to the diameter of neck of insulator plus half an inch (i.e. J inch at each end). Twist the double wire together at each end until the bottoms of the twists reach the neck of the insula- tor and then pass one wire of each pair in a clockwise direction round the neck and the other in a counter clock- wise direction. Twist the pairs together again when they meet until on bending upwards the tops of the twists just reach the conduc- x nr FIG. 45. Top groove binding. (These two wires must go round the conductor in the same direc- tion.) Now take 4 or 5 turns round the conductor with the short wire of each pair. Finally, take the other wire of each pair, pass them over the top of the insulator so that they cross each other and the conductor, and A finish off with 7. or 8 turns round the conductor. v. Terminating Small Conduc- tors (Fig. 46). Pass the conductor round the neck of the insulator and lay up the free end along the line part for 3 inches (for wires below -162 inch diameter, 2 inches will do). The bends in 5 Not less than 6 ins FIG. 46. Small conductor termination. 66 OVERHEAD POWER LINES TABLE VII. Lengths of Binding Wire Required, for Copper Conductors. Diameter of Nock of Size of Binding Length of Bindinp; Wire Enquired. Overlap when Layln'jf up Conductor. Conductor. Insulator or Sliackle. Wire, S.W.Cf. Termination. Side. Top. Binder. K. Ins. Ins. Ft. Ins. Ft. Ins. Ft. Ins. TllH. .136 136 3 14 4 8 7 7 162 162 3 14 6 8 7 6 7 193 193 3 14 7 9 8 8 3/-147 317 3 14 9 G 11 11 10 3/-18 388 3 12 10 11 11 12 7/-136 408 3 12 , 11 11 12 7/-166 498 3 12 . 12 13 14 7/-193 579 3 12 c 13 14 10 Approximate lengths of 1 Ib. of Copper Binding Wire are : 12 S.W.Q., 30 feet ; 14 S.W.G., 50 feet. The lengths naturally vary with the size of the insulator, and the exact figure should be determined by trial in particular cases, but the Table will assist when estimating. The table allows for a layer of binding wire on the conductor itself where in the groove, to prevent chafing between the wire and tho nock of the insulator. This is considered good practice in this country, and incidentally it luiljw to prevent burning of the conductor when a "flash-over" occurs. Alternatively copper tape may be used as a chafer, in which case some 20 to 30 % ICH.H binding wire will be required. the conductor at the point A must not be too sharp. Then bond out the free end at right angles, leaving sufficient length (not less than 6 inches) for connection to leading-in cable. Now pass a length of binding wire round the insulator, twist the two ends together and then, with the double binding wire, bind the free end of the conductor to the line part for the 3 inches overlap, finishing of by serving the single conductor for a length of 1|- inches. ^-Terminating Large Conductors. The common methods employed are shown in Fig. 47. H. V. Tensioning Insulators. Pour strengths of tensioning insulators are standardised for H.V. lines, viz. 400, 800, 1 200 and 2 400 lb. } but they can be obtained for loads up to 10 tons if required in special cases. Figure 47 illustrates four distinct types which are in use. Type " a "the SHACKLE Insulator is good mechanically and is quite suitable for L.V. work. It is also used on H.V. work for volt- ages up to 6 600 volt, but above that it becomes unwieldy in size. Type "6," the HEWLETT INTERLINKED type has been exten- sively used in the past and is a good design mechanically. INSULATORS 67 Type " c " is also an INTERLINKED type similar to type " b," but it is claimed that the porcelain is shaped so as to cause a better distribution of the dielectric stress. SCALE (a) SHACKLE TYPE 6.600 VOLTS , BOO POUNDS. (6) HEWLETT INTERLINKED DISC TYPE 1 1,000 VOLTS. 1400 POUNDS (TWO 6600 VOLT UNITS) (0) TWISS INTERLINKED DISC TYPE 1 1 000 VOLTS, 1400 POUNDS (TWO 6600 VOLT UNITS) (d) CAP AND PIN (OR METAL HOOD) TYPE, gZOOO VOLTS. 2800 POUNDS (TWO 1 1000 VOLT UNITS) FIG. 47. Typical high voltage tonsioning insulators. No cement is used in either of the above types and the porcelain is in compression. Moreover, if the insulator breaks, the wire is still linked mechanically and therefore does not fall. For these reasons some engineers prefer them to 68 OVERHEAD POWER LINES Type " d." This type, which is called the METAL HOODED, or CA: AND PIN, TYPE, will be seen to have the porcelain in tension and th< cement in shear, and the design is a radical departure from that whicl was rigidly adhered to in the early days, when the porcelain was nse< in compression only. But the porcelain of to-day is more uniformly reliable than it was in the past, and this design has been used sue cessfully for some years. Recent improvements in the methods o fixing the pin, e.g. the " split ring " method, have enabled thi type to be manufactured for working loads up to 8 tons. It i certainly by far the best design for very high voltages owing to tin uniform dielectric stress distribution. However, for the moderate high voltages used in distributioi work, the types mentioned may be considered to be equally reliabl and a choice made on the basis of first cost. ./Insulators of Materials other than Porcelain. Ex perience justifies the opinion that British porcelain has no supeiio as an overhead line insulator and it is now strongly fortified by B.S. Specification. But GLASS cannot be entirely neglected. It has been and sti is largely used on the Continent, and it is much cheaper than British porcelain. Up to 22 000 volts at least modern continental designs of glas insulator appear to be thoroughly reliable, and it is a pity that th manufacture of glass suitable for H.V. Insulators has not bee: seriously undertaken in this country. But both porcelain and glass are very fragile, and it is probabl true to say that more line breakdowns are due to broken or defectiv insulators than to all other causes put together. Owing to this drawback, many attempts have been made t produce a satisfactory substitute. Among such substitutes whic! have been placed on the market may be mentioned KALANIT: (Callenrlers Cable and Construction Co.), TELENDUKON (Thoma De La Rue & Co.) and EBONESTOS (Ebonestos Insulators, Ltd/ All these materials are tough and non-hygroscopic, and initially at any rate, they have the requisite dielectric strength. They appea to give satisfaction on telegraph and telephone lines and on powe lines up to about 6 000 volts, but they are more expensive tha porcelain. STEATITE, a naturally occurring magnesium silicate, has electric* properties equal to those of porcelain and very much higher ten si] and bending strengths. INSULATORS 69 Insulators of this material can "be made absolutely puncture proof. They are reputed to stand up well to stone throwing, but at present their high cost limits their use to special situations. Another material which shows promise is FUSED BASALT, a dark-coloured rock of volcanic origin which can be moulded to almost any desired shape at a temperature of 2 300 F. It is claimed to have all the advantages of porcelain, and in ad- dition it has a strength of about 18 tons per square inch both in tension and compression, and possesses the remarkable property of resealmg itself when punctured. Moreover, it is said to be cheaper than porcelain. It is understood that a number of insulators of this material are on trial in this country. In the present state of development none of these materials can be recommended to replace porcelain except steatite, but they might be tried in sections of a line subject to trouble from stone throwing. There is a fortune awaiting the inventor of a material which has all the electrical advantages of porcelain, with its durability but without its fragility. 70 CHAPTER V. CROSS ARMS AND INSULATOR BRACKETS. THE methods of supporting the insulators and securing them to the pole afford much scope for ingenuity. It is axiomatic that the poles should be cut and drilled as little as possible after creosoting. The sapwood only absorbs the creosote impregnation, and it is therefore important to avoid penetrating the heartwood when cutting slots. All slots, holes, etc., cut in poles should be painted with a hot creosote tar mixture (2 creosote, 1 coal tar). Table VIII. gives particulars of some useful angle and channel sections. For pin insulators, the channel section with the web ^ertical is better than the angle owing to the greater depth for securing the pin, but the angle is much stronger, weight for weight, in the direction of the line, and can be adapted for the fitting of standard insulator pins by inserting an oak block. Power engineers do not favour the use of timber cross arms, but their prejudice appears to be unjustified when one considers the fifty years' experience of the Post Office with arms of oak and other hardwoods. Wood impregnated with bakelite varnish, known as BAKELISED WOOD, has recently been introduced by a French firm. It is stated to have a tensile strength three times that of the untreated wood. and a very high dielectric strength. It appears to be a very promising material to use for cross arms and insulator pins (and possibly for the insulators themselves) in situations near the coast where porcelain insulators give trouble due to the salt-laden air when fitted to steel pins and pole fittings. All pole ironwork should be galvanised when possible. After ' immersion for twenty minutes in a solution of hydrochloric acid in ' water (equal parts acid and water) the parts should be immersed in a ' bath of pure molten zinc, and then placed at an angle to set. When ironwork is made up locally galvanising may not always CROSS ARMS AND INSULATOR BRACKETS 71 TABLE VIII. Particulars of Steel Sections. Material. Section. Weight per ft. Ib. Area A sq. in. Moment of Inertia, J. Strength ModuluSj Z. Uaclius of Gyration, Jc. *2 X 1-| if 1 1 *2 X 1J E J 3-84 1-125 / 0-24 (^ 0-65 0-25 0-65 0-465 0-760 3 X IJ [t 1 3 V . 1 1 l___^. j 4-60 1-352 ( 0-261 | 1-823 0-255 1-215 0-439 1-161 S X i* 1" *3J X 2 . [T \ r / 6-75 1-986 { 0-713 ^ 3-701 0-526 2-115 0-599 1-370 z *3|- X 2 "t o 4 X 2 [T | 4v> O _._l__._ *^ 7-09 2-085 ( 0-703 \ 5-063 0-502 2-532 0-581 1-558 X ^ j----- 5 X 2| [T 1 10-22 V 3-006 / 1-641 | 11-873 0-95 4-749 0-739 1-987 5 X 2|- \ 6 X 3 [T | 12-41 3-650 / 2-825 [21-271 1-339 7-09 0-880 2-414 X 3 LI. itxHxt r i 2-34 687 f -134 1 -056 128 441 286 14 x ij x i JT ] I If X If X -3 ft^ | 3-26 960 f "255 108 21 515 335 If X If X -3 / -T : J L C5 2 X 2 X -3 ft" | 2x2 X -3 X" > 3-77 1-110 f -392 j -164 28 594 384 1 2| X 2i X -375 fj~ ^ 5-90 1-735 / -962 1 -402 55 745 482 > 2J X 2J X -375 / p Ji ' j L W 3 X 3 X -375 it" 1 7-17 2-110 f 1-72 \ -712 81 903 581 3 X 3 X -375 ,.f~ J I 4 X 4 X -425 [j ^ 10-95 3-220 f 4-752 1 1-953 1-603 1-215 779 4 X 4 X -425 X" * I The lesser values of J and fc must be used for struts. With, the exception of those starred, the above particulars have been, extracted by permission of the British Engineering Standards Association from British Standard Specification No. 6 for rolled steel sections for structural purposes. (See p. vii.) 72 OVERHEAD POWER LINES be practicable, in which case the parts should be immersed in bath of hot gas tar (300 F.) thinned with a little gas oil, until t iron attains the temperature of the bath, and then placed at an an to set. On erection a couple of coats of tar varnish or bitumasl solution should be given. For simple rectangular sections, if b = breadth and d the depth, =~ =- 12 6 For circular sections of diameter D, , TiD 4 TrD 3 , J== -6T Z - -32 and Eccentric Loading. For eccentric loading producing con bined bending and direct stress, if T and C represent the direc tensile and compressive loads in lb., M the bending moment i Ib.-inches, Z the strength modulus in inch units and A the area c section in sq. ins., then the following formula will give the maximun tensile and compressive stresses in the lb./ sq. inch, (i) When DIRECT STEESS is TENSILE, T M ,____ Jc ~Z c A" (ii) When DIEECT STEESS is COMPRESSIVE. f-^+^ Jc ~~ A ^ Z e __M __C Jt ~ Z t A' In the absence of precise information, the following values for ultimate stresses, etc., of ordinary mild steel, Norwegian red fir and oak may be assumed in calculations. Special steel may be obtained of more than double the strength of ordinary mild steel, and is generally preferred for insulator pins. The figures for timber are good average values. CROSS ARMS AND INSULATOR BRACKETS 73 TABLE IX. Ultimate Strengths of Mild Steel* Oak, and Red Fir. MILD STEEL. (i) Ultimate Tensile Stress, Tensile . 65000 Ib. /sq. in. (ii) Compressive 55 000 ,, (iii) Shear . . 50000 (iv) Elastic Limit, Tensile or Compressive 36 000 ,, (v) Shear . . . 27000 (vi) Modulus of Rupture (Bending) . . 60 000 (vii) Elasticity, E . . 30 X 10" ENGLISH OAK. (i) Ultimate Stress, Tensile (a) Parallel to grain ... 10 000 (b) Perpendicular to grain . . 900 ,, (ii) Ultimate Stress Compressive (a) Parallel to grain . . . 8000 ,, (b) Perpendicular to grain . . 2 000 ,, (iii) Ultimate Stress, Shear (a) Parallel to grain . . . 800 ,, (b) Perpendicular to grain . . 4000 ,, (iv) Modulus of Rupture (Bending) . . 9 000 Elasticity, E . . 1-2 x 10 NORWEGIAN RED Era. (i) Ultimate Stress, Tensile (a) Parallel to grain . . . 8 000 (b) Perpendicular to grain . . 500 ,. (ii) Ultimate Stress Compressive (a) Parallel to grain ... 6 000 (b) Perpendicular to grain . . 1 500 (iii) Ultimate Stress Shear (a) Parallel to grain . . . 500 (b) Perpendicular to grain . . 4 000 (iv) Modulus of Rupture (Bending) . 7 800 (v) Elasticity, E . . . 1-2 x 10 * Although referred to as " iron " in the colloquial sense in various places throughout the book, " Mild Steel " (-2 % carbon) is the material most commonly used in overhead line work. Good quality wrought iron, however, is very little inferior in strength to mild steel. Selections from the many types of pole ironwork met with, are shown in Figs. 26 to 37, and to assist the reader to form opinions as to their respective merits and demerits some rough calculations are given. In practice it may not be usual to calculate very closely, neither is it possible to be very precise, but from an inspection of some types of fitting in use, the writer is of opinion that there is sometimes a lack of appreciation of the stresses to be dealt with and that it. is fortunate for the engineer concerned that the hypothetical loading conditions are seldom experienced. 74 OVERHEAD POWER LINES 1 The importance of preventing deformation of the ironwork und stress must be emphasised. Any appreciable movement will alt the stress distribution in the insulator and may lead to fracture the porcelain. The designs are checked on the basis of the maximu (lateral) horizontal and vertical loadings, but there may always 1 a certain amount of unbalanced longitudinal pull. The latter mi reach large values at angles since, as mentioned before, the wir cannot be at right angles to both spans simultaneously. Moreove reversals of stress occur when the direction of the wind changes ar the fitting of struts, ties and bolts is never perfect. Eor the above reasons it is desirable to keep the working stressi low. It is proposed to allow a factor of safety of 2-5 on the elast limit of iron and steel in tension, shear and bending and on the cri] pling load for struts. Fo&the timber a factor of safety of 3-5 on tl ultimate stress will be taken. All struts will be assumed to be hinge at both ends, although this may appear to be pessimistic in some case In accordance with the above the Working Stresses will be take as follows : 3 Working Stresses in Mild Steel. I. DIRECT TENSION AND BENDING.- Safe Working Tensile Stres . 36000 =^/< ^ -7TF- = U 40 lb ' / SC I- m ' jL'O II. DIEEGT COMPRESSION (STEUTS). In the following I = effec tive length of strut in inches ; <7 = moment of inertia in inch units "k = radius of gyration in inches ; A = area of cross-section i: square inches. (a) For values o/y > 100. K Euler's formula for a strut hinged at both ends will be used. 7T n v i i D Crippling load : B = J = AW. Safe Working Compressive Stress / - 10 30 1Q6 A7 2 - 12 x 1Q7 /c ~ I* . 2-5 . A (b) For values of - from 60 to 100. CROSS ARMS AND INSULATOR BRACKETS 75 The following empirical straight line formula will be used (see B.S. Spec. No. 61924) : Crippling Stress =/ c = (46 000 - 166 7 ) Ib. / sq. in. \ KJ .-. Safe Working Compressive Stress = l8 400 66 Ib. / sq. in. (c) For values ofj < 60. 1 When j = 60-7, the straight line formula gives / c = 14 4-00 A* Ib. / sq. in., which is the maximum value allowed in tension. Now it is contrary to experience to find members stronger in compression than in tension ; so for short struts f c will be limited. to 14 400 Ib. / sq. in., although the straight line formula will indicate a larger value. III. SHEAR. Safe Working Shear Stress Ib. / sq. in. Working Stresses in Bolts. 27000 2-5 = 10800 TABLE X. Particulars of Bolts (WMtworth Threads), Overall Diameter, ir.s. Total Area, sq. ins. Diameter at Bottom of Threads, ins. Area at Bottom of Threads, B<I. ins. 5 196 393 121 625 307 509 204 75 442 622 304 875 601 733 422 1-00 785 840 554 It is recommended that the Factor of Safety for bolts should be 2-5, also based on the elastic limit, since although the elastic defor- mation of bolts may be negligibly small as far as the shape of the fitting is concerned, there is always an indefinite tensile and , tor- sional stress in the bolts due to tightening up the nut. The tensile load might easily be 3 000 Ib. which means in a f-inch bolt a tensile stress of = 9 900 Ib. / sq. in. at bottom of threads. *o04 The stress may be 15-20 % greater than this due to torsion. The necessity for large washers on bolts through timber will be apparent. 76 OVERHEAD POWER LINES The bearing pressure under washers may be taken as 25 % greater than the greatest permissible compressive stress allowed in the general design, e.g. if the tensile load on the bolt is 3 000 lb., the area of washer should be 3 000 = 5-6 sq. ins. 1 500 A 3 in. X 2 in. X \ in. washer will meet the case. Bolts through poles should always be a driving fit. To ensure this the hole should be drilled with an auger TIT inch only larger than the bolt. Coach Screws. Coach screws are found to be very useful in pole line work when used intelligently. To fix the screw a hole should be bored a sixteenth less than the overall diameter of the threaded portion of the shank, and if an appreciable part of the unthreaded portion of the shank penetrates the pole provision should be made for it by enlarging the hole to the requisite depth. Driving the screw part of the way with a hammer, as is frequently done, always lessens the holding power. A number of experiments have been made to test the holding power of coach screws. Allowing a F. of S. of 3-5 to 4 and rounding off the figures the safe holding power when inserted across the grain in Norwegian Bed Fir, per inch of penetration of thread may be taken to be : -| inch diameter, 300 lb. ; inch, 350 lb. ; f inch, 400 lb. These figures apply to any direction of pull, providing that the coach screw penetrates the pole up to a point not less than f inch under the head. That is to say, the thickness of the material secured to the pole must not exceed f inch . If it does the holding power is reduced when the pull is non-axial, although there may be the same length of penetration of thread into the pole. Working Stresses in Norwegian Red Fir. The elastic limit for timber is approximately 75 % of the ultimate stress, but there is not the same precision about the figure that there is about the elastic limit of steel. It is proposed to base the Factor of Safety on the -ultimate stress in all cases. Bearing Pressure of Round Bolts on Timber at Bolt Holes. I. LOAD AT EIGHT ANGLES TO FIBRES. In this case the bearing area may be based on the full diameter of bolt (d). CROSS ARMS AND INSULATOR BRACKETS 77 II. LOAD PARALLEL TO FIBRES. In this case it can be shown that the effective width of bearing surface is only about six-tenths of the diameter (i.e. Q'6d), e.g. with a f-inch bolt, the safe maximum working load per inch length of bolt r , 1 500 X -75 In case I. = ^-= = 321 ID. o'O , TT 6 000 X -75 X -6 , In case II. = ^-= = 771 Ib. o-5 It might be noted in passing that in Case II. there is a load at right angles to the fibres equal to about one-tenth of the longitudinal load. This transverse load tends to split the pole and has to be taken into account in designing timber joints, but it is not likely to be of importance in connection with pole fittings. Case III. LOAD INCLINED TO FIBRES. In this case the simp- lest procedure is to resolve into two components parallel and at right angles respectively to the fibres. Continuing with the f-inch bolt, if the load per inch length = P at an angle of 30 with the fibres, then the component along fibres = P cos 30 and at right angles thereto = P sin 30. 771 As far as longitudinal strength is concerned P may be ^-r = 771 ^91 391 4^~ = 890 Ib., but it is limited to 4^- = -= = 642 Ib. by the 866 sin 30 -5 J transverse strength. We will now consider the various fittings illustrated : In the following, W = dead weight of wire plus ice ; P^ = P -f- / f)\ T m (2 sin - ) = P-)- T m oc, in which P l = total lateral load on \ 2/ insulator, P lateral load due to wind pressure, T m = maximum safe tensile load on conductor, 6 = angle of deviation of line and n cc = 2 sin - which is plotted in Fig. 41, page 60. 2i Table XL gives values of W and P for the standard lengths of span suggested in Table XIV., page 106, which will be kept in mind throughout, as well as Table VI., page 61, giving the maximum deviations in the line which are permissible, using standard insulator pins. 78 OVERHEAD POWER LINES TABLE XI. Values of W, P and T m for Standard Spans erected to E.G. Regulations for H.V. Lines. Conductor. Standard Span, feet. TF per foot. Ib. P per foot, ( Ib. TF (total), Ib. P (total), Ib. T , m' Ib. 162 200 331 608 66 122 633 193 250 379 629 95 157 874 3 / -147 280 521 710 146 199 1457 3/-18 315 654 758 206 239 2125 7/-136 335 762 771 255 258 2935 7/-166 350 999 832 350 291 4265 7 / -193 335 1-244 885 417 297 5635 Fig. 26 (page 45). Cross Arm. Suppose wind to be blowing from left to right (Fig. 26 (a)) and assume bending at section " aa " at centre of arm (i.e. neglect support given by slot). First consider the two sides independently. (a) Right Side. It is a case of eccentric loading A = 2- 085 sq. ins., Z t ~= 2- 532 inch units (Table VIII., page 71), M=24F+8P 1 . 3-- Channel a* W FIG. 26 (a). Maximum value of P x = 800 Ib. when the stronger standard pin is used, therefore for 7 / -193 conductor on standard 335 feet span ft 800 24 x 417 + 8 x 800 2- 085 ^ 2- 532 = 380+6 480 = 6 860 Ib. / sq. in. -,. = stress at outer edge of top flange. Similarly /,, the stress at outer edge of bottom flange, 380 = 6 100 Ib. / sq. in. 6480 CROSS ARMS AND INSULATOR BRACKETS '(&) LeftSide. 79 . A z; 24F = 6 400 10 000 = - 3 600 = 380 1 420 = 1 040 Ib. / sq. in., i.e. the stress at the outer edge of the top flange is tensile and equal to 1 040 Ib. / sq. in. At the outer edge of the bottom flange f c jfc= 380 + 1 420 = 1 800 Ib. / sq. in. We have, therefore, Maximum tensile stress in top flange = 6 860 Ib. / sq. in. and maximum compressive stress in bottom flange = 6 100 Ib. / sq. in. These stresses are well below 14 400 Ib. / sq. in., the maximum safe working value. The maximum unbalanced tensile stress in the top flange = 6 860 - 1 040 = 5 820 Ib. / sq. in. = 5 060 + 760 = 8 * 2 f gL* 2 + 380 X 2, and the unbalanced compressive stress in the bottom flange = 6100-1800 = 4300=5060 - 760 == 8 X 8 ^ X 2 - 380 X 2. a* Qua The bending moment due to these forces (viz. 5 060 X 2- 532 = 12 800 Ib.-ins.) must be dealt with by the slot. Strength of Slot (Fig. 26 (&)). Assume pole diameter D= 8 ins. and depth of slot b = 1-|- ins. 4 43 Area of segment = -^bd = Q- X ^ X 3-12 = 6-24 sq. ins. 80 OVERHEAD POWER LINES Since 6 < the arc may be assumed to be a parabola for whicl 2 j;*w. and Now tlie maximum compressive stress on the timber in the slot for the values of If and P t being considered,, 2x 417 12800 O.OAIT, / = -6^+-M- = 342 lb - / but/ c must not exceed 1 715 Ib. / sq. in. * .. P! must be reduced from 800 Strength of Arm Bolt Fixing (Kg. 26 (c)). The bolt pivots about the centre of the 6-| inch of pole which remains after the 11- inch slot has been cut. If f c is the maximum compressive stress in the timber due to the bending moment on the bolt, we have Bolt dia. = 0-75 inch ; .-. A 6-5 X~75 = 4-88 sq. ins. ; G = 2Pj Ib., and M = P x X 2 X (3-25 -j- -12) = 6-74?!, (If a 3 in. X 3 in. oak arm were used M = P! X 2 X (3-25 + 1-5) = 9-5P x and the method of fixing therefore much weaker.) The compressive load diagram is shown in Fig. 26 (c). The total load on timber on each side of axis, if / B = maximum, stress due to bending moment, = 3-25 x -75 X ~ 1- 217 / 6 , */ a CROSS ARMS AND INSULATOR BRACKETS 81 The centre of pressure of this triangular load 2 = 3-25 X = 2-16 inches from axis. A .-. Moment of resistance = 1-217 X 2-16 X 2 X / & = 5-25/ 6 . Whence Z = 5-25 inch units. Substituting known values in the stress equation we have 2P X 6-74P! ~~~~ J n"r ~T~ P SI? ""~~~ 1' 1500 3-5 429 Ib. / sq. in. / at right angles to fibres must not exceed .-. P l is limited to = 252 Ib. as far as the timber is concerned. Now, consider the bolt. Fia. 26 (c). The safe moment of resistance of a f-inch bolt, if we base the F. of S. upon the elastic limit _^3^ _ OT . 14 400 = 597 Ib.-ins,. 32 32.64 Assuming no flexure of the bolt, the maximum bending moment on it occurs at the centre and is equal to half the total bending moment, .e. -, = X 252 = 84-9 Ib.-ins. As this is too great for the bolt, the total bending moment must be reduced to 597 + 849 = 1 446 Ib.-ins., and P 1 1446 to 6-74 = 215 Ib. 82 OVERHEAD POWER LINES But tliis is a pessimistic result, since if tlie nut is tightened up so as to produce a tension of 3 000 Ib. in the bolt, the frictional force between the arm and the pole is quite considerable. Assuming a coefficient of friction of iron on wood of 0-3, this force = 3 000 X -3 = 900 Ib., and allowing a Factor of Safety of 3-5 it will enable P x to be increased by x 5-5^ = 124 Ib., making 0*0 x & o'oi a total of 215 + 124 = 339 Ib, If, on the other hand, the nut is drawn up lightly we may base tlie Factor of Safety upon the ultimate strength of the bolt and its moment of resistance would then be 597 X .- = 995 Ib.-ins. 14 400 and therefore the timber will give before the bolt, If the length of the spanner is not too great (12 to 15 times tlu diameter of the bolt) bolts should always be drawn up as tightlj as possible, since the frictional forces help both the timber anc the bolt itself and so materially increases the holding power. It will be seen from the above figures that the simple slot fixin, is suitable for all sizes of conductor and span lengths given in Tabl XIV., providing the line is quite straight, but for larger lateral load such, as arc experienced at angles, some form, of reinforcement i necessary. Wo will first consider the effect of adding diagonals as show dotted in Fig. 26, page 45, ' : - Diagonals. In order that the diagonals may be effective, it desirable that there should be no deflection at their points of attacl ment to the cross arm.. The calculations should, therefore, real be based upon deflections rather than upon moments, but it simpler to consider the latter and the result so obtained is sufficient accurate for our purpose. Assuming the dimensions shown in Fig. 26 (d) and taking m ments about " a," we have, if C is the compressive load in t: right hand diagonal, 16= W X 24+ P t X 8. Taking W = 417 Ib. and Pj. = 800 Ib., we get G = 1 450 Ib. Sir larly for the left-hand diagonal 'JLx 16= FX 24-PjX 8 andC= 318 Ib. CROSS ARMS AND INSULATOR BRACKETS 83 Now it will be impracticable to use anything less than li X 1 X |- angle, owing to the end fixing requirements. For this section I 17 j -j- = 58-5. As this is less than 60, the safe working stress = 14 400 Ib. / sq. in. , .-. Max. safe working load = / x A = 14 400 X -687 = 9900 Ib. The section is therefore amply strong enough. In accordance with the above the load on the slot would be upwards and equal to 1 025 + 225 417 417 = 416 Ib., but as explained later, the load on the slot is much more likely to be vertically downwards when the fitting has settled down. Now consider the strength of the bolts at " a " and " de" Tak- ing " de " first, and assuming that the vertical load on the timber is uniformly distributed, the safe working load = 1 715 X -6 X 8 X P Boo -p-fj^Boo f =^ 8" < 8" > * iz ^i I rt 800 aoo ^ |a \ ' 22-5 f A. nn / |\ \ 12" ffc5 ,t ^ . . "9 X\ I //^~ \A/3 A 1 7" x v TI / . 2,2.5 \ i*h r /IQZS W-4-lT hTHTI "* ( 225' I025 FIG. 26 (d). ; 75 = 3 080 Ib., so there is ample strength as far as the timber is concerned. The maximum bending moment on the bolt (f-inch diameter) = 1 025 X 2-125 = 2 180 Ib.-ins., and its moment of resistance is 1 490 Ib.-ins. at the elastic limit. The frictional force between angle iron and pole may be assumed to increase this by (3 000 - 1 025 - 225) X 4 X 2 -4- 2-5 =. 560 Ib.-ins., making a total moment of resistance of 2 050 lb. : ins. only, which is quite inadequate. Actually, however, this is a pessimistic result, since although the diagonals should be capable of dealing with the total load initially, flexure of the lower bolt will enable the arm to take its share of the load, and for this reason the maximum bending moment on the bolt should not exceed about one half the value calculated above. More- over, when the bolt bends, the load distribution on the timber in the 84 OVERHEAD POWER LINES bolt hole is altered, and instead of remaining uniform it becomes greater near the surface of the pole. For a given vertical load, this means a shift of the centre of pressure of the reaction towards the point of application of the load and this reduces the bending moment on the bolt. All things considered, a f-inch bolt will; be found quite suitable for the requirements in this case. With regard to the bolt at "a," without the diagonals the load on this bolt would be 1 600 Ib. With the diagonals, the load is seen ,,,.. to be considerably greater, and it is there- " |X 8 rap ' fore quite useless to add diagonals, without strengthening the arm fixing, for which & . n ' 26 (e] ^ e ma ximum sate load was shown above to be 678 Ib. only. The best way to do this is to put a 1 strap round the pole, as shown in Fig. 26 (e). In this way the diagonals will serve a useful purpose for loads ' up to P x = 800 Ib. on pin insulators. Above this value, tensioning insulators must be used and then P l acts along the axis of the arm, thus rendering diagonals really unnecessary, although it might be desirable to retain them. By fixing the arm to the pole by means of a strap in this way the lateral working load at angles is limited only by the buckling strength of the pole. Double Arms. Double arms may sometimes be desirable at angle poles to maintain sufficient clearances between conductor and pole on outside of angle. In this connection, the reader may be reminded that to maintain a spacing of sc. feet throughout the span, the distance between insulators at angle poles must be increased to /g - T, 6 being as before the deviation in the line. This is one of the reasons why " H " poles are often used at large angles, another being the limit imposed by the buckling stress of the pole. Assuming f-inch bolt and 8-inch pole as before and two l-|~inch slots, the safe working load on the timber in the bolt hole = 5 X -75 X 429 = 1 610 Ib. The bending moment on the bolt with this load For reasons explained above this may be considered satisfactory for a value of P t of about 800 Ib. CROSS ARMS AND INSULATOR BRACKETS Cap Fitting (Fig. 26 (/)). It must be here pointed out that a distance of 9 inches above the pole, as shown in Fig. 26, is rather on the small side. 10 inches is about the minimum practicable figure, using stan- dard 11 000 volt pin insulators. To simplify the considera- tions, assume a theoretical design with the dimensions shown in Fig. 26 (/). (1) First consider the 'part cdfe. The BENDING MOMENT acting on this part = 7 P Ib.-ins. approx. The MOMENT OP RESISTANCE to deformation will be four times that of one 2 in. X f in. section iff X 36 000) = 4( 2X 6 9X 6 3 f ) = 4 X 1 090 \ \ -4 : -42" 3 f2" P > 6 e. ' 1 , "'- ^ ( i i h : 1 %"t : b d FIG. 2C \6 = 6 760 Ib.-ins. To this might reasonably be added the moment of resistance of the two -inch insulator bolts = 1 210 Ib. -. P at elastic limit 6760 1730 (2) Now consider the other part abed. Between ab and cd the fitting is a girder without a web, and it is, therefore, relatively weak as far as shearing is concerned. If the connections at e and d were pivoted, the shearing load P acting along the line cd would produce bending moments in the two flanges independently, and they would share the load equally if the lower flange were supported at b. But actually the unsupported length of the lower flange is 6 inches and of the upper 2 inches only and as the deflections are proportional to the cubes of the lengths it will be clear that the upper flange takes most of the load. It is, therefore, suggested that the MOMENT or RESISTANCE to deformation is the sum of the moments of resistance of the sections at a and o ~ 2x1 690 Ib.-ins. The BENDING MOMENT due to the shearing load 2P Ib.-ins. .-. P = 1 690 Ib. 86 OVERHEAD POWER LINES In the practical design the effective lengths of ac and ce depend upon the fitting of the cross strap, but the above treatment is suf- ficient to indicate roughly how the strength will be affected by alter- ing the dimensions. Experimentally, the fitting shown in Fig. 26, with a single rivet on each side was found to have an elastic limit of 1 300 Ib. approx. and therefore a maximum safe working load of 520 Ib. The following points may be noted : (1) The rectangular shape is necessary in the example chosen in order to take double insulators, but sloping the sides inwards as shown in Fig. 30 makes a better job with single insulators. (2) The cross strap should always be bent downwards where riveted to main member so as to reduce the effective length of ac to a minimum. Two rivets on each side placed diagonally will enable the working load to be increased a little. (3) Although it will slightly decrease the value of the safe working load, the distance ce may with advantage be increased an inch or so in order that insulator pins of the same length may be used on both cap fittings and cross arms. (4) |- .inch bolts are shown, but 3-J in. X -| in. coach screws will do equally well. (5) The upper bolt must be at a sufficient distance from top of pole to avoid crushing the timber. This can be checked as explained on page 80. 4 inches is about right, but it is advisable to plane the pole a little so as to get a fiat bearing surface 2 inches wide. Fig. 27 (page 45). Consider Section " aa " (Fig. 27 (a)). A ~r Z, _f + T TO cc lBW+1(P+T n cc) ~ A ~r Z { A I- 352 sq_. ins., Z = -255 inch units. 'Introducing values for -193 conductor on 250 feet span we get 158+_87a 18 X 95 + 7(158 + 874 oc) / = '* ~ I- 352 -255 = 11 167 -f 24 645 oc = 14 400 .-. oc = -13 and 6=1 (Fig. 41). CROSS ARMS AND INSULATOR BRACKETS 87 This fitting cannot be used for conductors larger than -193 on the standard spans considered. It is a simple design but makes very poor use of the material. Fig. 28 (page 45). We will consider the top left-hand fitting of Kg. 28 which pro- vides sufficient clearance from the pole for a 22 000 volt line. Dimensions assumed in calculations are given in Fig. 28 (a). The diagonal may be taken to be a strut concentrically loaded, but the horizontal members are subjected to eccentric loading. In this case we will base the calculations on 7 / -136 conductor in the straight on a span length of 335 feet for which W = 255 Ib. and P 1 258 Ib. The direct loads, as determined below, are shown in Fig. 28. tfia. 27 (.). FIG. 28 (a). Load in Diagonal. If C = Compressive load in diagonal, X 20 = 21-5F X (i) No WIND. P l = 0, W = 255, ...(7= -1-27 X 255= 324 Ib. (ii) WIND OUTWARDS. C = 1-27 X 255 + 415 X 258 = 324 + 107 = 431 Ib, OVERHEAD POWER LINES (iii) WIND INWARDS. = 324 107 = 217 Ib. Strength of Diagonal. I 22-5 = 312. k -289 X -25 .. Maximum Safe Working Compressive Load __ 12 x 10 7 x_A. __ 12 X 10 7 X 1-5 X '25 ~ 312 2 = 462 Ib. FIG. 28(0). Load on Horizontal Members. The horizontal members make an angle = cos" 1 . 96 with the direction of P t . It is proposed to neglect this. (i) No WIND. Direct Load = C X L 2 jf = -68TF = -68 X 255 = 173 Ib. 19 Bending Moment = l-5Tf = 1-5 X 255 = 382-5 Ib.-ins. 1 _ . 7 2 2 x " 25 x 22 4- t) o CROSS ARMS AND INSULATOR BRACKETS 89 (ii) WIND OUTWARDS. Direct Load 19 1 = ^(324 + 107) + 258 ' = 173 + 57 + 258 = 315 + 173 = 488 Ib. (tensile). Bending Moment = 1-5W -j- 7 PI = 1-5 X 255 + 7 X 258 = 2 200 Ib.-ins. T_ M '''^~A + Z t 488 , 2200 - A0on , , 1_ _ 7 088 Ib. / sq. m. 3 Maximum permissible Tensile Stress = 14 400 Ib. / sq. in. (iii) WIND INWARDS. Direct Load = 315 173 = 142 Ib. (compressive). T FIG. 28 (c). Bending Moment. In this case, the members are curved, and the maximum Bending Moments occur at the points furthest away from the straight lines joining the two ends of the members, i.e. M = C X 2-4 Ib.-ins. (Fig. 28 (c)). Z on weaker axis = 2-^- o bd* 2.2. 6 _ 24' , _ 142 142 x 2-4 .-./ c _ 1 + ^ 24 = 142+8 180 = 8 322 Ib./sq. in. (Maximum permissible Compressive Stress = 14 400 Ib. / sq. in.) It may be noted in passing that if additional coach screws were 90 OVERHEAD POWER LINES placed at the points nn, where the straps are tangential to the po. they would become simple concentrically loaded struts of leng 16 inches and very much stronger to resist wind loading inwar towards the pole. It will be seen from the above that the fitting is not suitable f conductors larger than 7/ -136 unless the span length is much r duced, and it can only be used with 7/- 136 if the line is qui straight. If the calculations are repeated for 3/-18 conductor will be found that an angle of about 6 degrees can be negotiate! and of course, with the smaller conductors, much larger angles ca be dealt with. As it is the relative weakness of the diagonal which is the limitii: factor, the fitting can be strengthened at very little cost by increasir the section of this member. The buckling load of a 1\ inch X $ inc section is more than double that of the 1| inch X J inch section use< Actually, the ends of the diagonals are more nearly fixed tha hinged and there is considerable friction between the horizonfe members and the po]e when the diagonal has to support the greate; load (i.e. when the wind blows outwards). Also, the diagonals ma possibly take some of the direct load P i} which would tend to reduc the compressive load in it when the wind blows outwards. These factors are comforting, but should not be made use of i the design. Strength of Coach Screws. The coach screw supportin the greatest load is that at the bottom of the diagonal. The vertics 19 component of the maximum load on the diagonal is 431 X 7^73 = Zji'O 365 Ib. The length of thread on a 3-|-inch coach screw exceed 2 inches and the safe working load is therefore at least 300 X 2 = 600 Ib. This neglects the help given by the frictional resistant between diagonal and pole, so the holding power is ample. This is a very sensible and cheap fitting and is easy to fix. I may be necessary to fit bird guards in some areas. As stated before the calculations refer to the top left-hand fitting on Fig. 28. Th< other two fittings are somewhat stronger. Fig. 29 (page 45). Fig. 29 is a weak design, and will only be briefly considered, The maximum safe MOMENT OF RESISTANCE TT.tZ 3 / Trxl X 36000 , ,, K1 _ . ^ 3Tx^B = 32 X 2-5 = 1 415 lb '- ms - CROSS ARMS AND INSULATOR BRACKETS .93 When the wind is outwards and the conductor in the side groove nearer the pole, the BENDING MOMENT = 6-5TF -f- 13Pj [Kg. 29 (a)]. Introducing values for the smallest conductor, -162 on a 200 feet span we get 6-5 X 66 + 13 XI22 =- 2 015 Ib.-ins. (The shearing and direct compressive loads on the section are negligible compared with the bending load.) This type is, therefore, ruled out altogether for the span lengths under consideration. It is really an elongated insulator bolt, and its use in H.V. work must be limited to 6 600 volt lines, with small Channel, FIG. 29 (a). PIG. 30 (a). 193 conductors on short spans. It may be used with -162 and copper on span lengths of 130-150 feet in straight runs only. For L.V. lines, in which the spans are usually short, the hypo- thetical loading conditions and the required pole clearances are both less, the type will be found useful. Fig. 30 (page 46). Side Fitting. It will be found that the following calculations (similar to those for Fig. 29 (a)) determine the strength of this fitting. Providing that the cutting and shaping at the lower end is reasonably well done, the strength of section is increased when opened out. OVERHEAD POWER LINES Fia. 32 (a). CROSS ARMS AND INSULATOR BRACKKTS 0tf The MOMKNT OF RESISTANCE of .section nt the point where the web is cut away .from the flanges x -520 - 7 580 Ib.-iiw. The BENDTNO MOMENT on this section = 10 >f-|- 157'j. [Fig. 30 (4J. Introducing values for 7/006 on a 350 foot span, we. have, in straight runs 10 X 350 + ]5 X 291 7 8(55 Ib.-ins. Thin is somewhat high, but as we have, been conservative as re- gards working stresn, the fitting might be considered about right. With 3 / Id 7 on, 280 feet spans, we, have 10 X 'MO H~ J GlOO + 1 457 X 2 Bin -- 7 580, \ A/ whenc-o ? 8, that is, the fitting is suitable for an angular deviation in the. lino of <S degrees. This is a simple and economical fitting. Cap Fitting. ....... This may be cheeked HH explained on, page 85, It is stronger than the side fitting. The, calculations for the remaining fittings will bo left as exercises for the rea4er. Fig. 32 (</) shows the approximate loads on the. various members of the, CaJ lender side pole fitting illustrated in Fig. 32 (single, insu- lator) with an outward wind load on insulator of '100 Ib, and a dead weight load (conductor and ice) of 200 Ib. 94 CHAPTER VI. SIMPLE WOOD SUPPORTING POLES. THERE are three types of pole to consider, viz. (a) SUPPORTS, (6) ANGLES, (c) TERMINALS. SUPPORTS are poles on which there are normally no longitudinal forces and which have, therefore, merely to support the wires and resist lateral loading due to wind pressure. Unbalanced forces at angles and terminals are dealt with by stays and struts. Single wood poles will be found suitable for most of the lines under consideration and attention will be confined to them in this chapter. Poles of NORWEGIAN RED FIR are standardised (B.S. Spec. 189 1921) and used very largely in this country. If felled at the correct season of the year and properly seasoned and creosoted they have a useful life of upwards of 40 years. It is specified that the polos shall retain their natural butt. ' The Regulations, however, do not pre- clude the use of other species of timber, and in this connection the " Cobra " method of impregnation (Cobra Limited, 30 Norfolk Street, Strand, W.C. 2) may be referred to as it can be applied to almost any kind of timber and to standing poles as well as to new poles before erection. The preservative is a water solution of 85 % sodium fluoride and 15 % sodium dinitro-phenate and it is injected by puncturing the pole at various points on its circumference by a special^ apparatus. To Determine the Size of Pole Required. Notation (see Fig. 48). L = Overall length of pole. H = Height from ground to point of loading (the point of loading is the centre of pressure due to wind load on all the con- ductors and wires carried on the pole). H V ft FIG. 48; ,D SIMPLE WOOD SUPPORTING POLES 95 A = Length of pole above point of loading. Ti = Length of pole buried in ground. D = Diameter of pole at ground level. All the above dimensions in inches. P = Wind load on wires in pounds. The BENDING MOMENT on the pole at ground level due to wind load on wires The WIND LOAD ON POLE (neglecting taper) _ _D(H + A)8 -P- 144 LD -> H 4- A which may be assumed to act at a height of = inches. 2i BENDING MOMENT DUE TO WIND LOAD ON POLE D . (R + A} z . 8 . = 2.144 lb -- 1M - TOTAL BENDING MOMENT 2 . 144 The MOMENT OE RESISTANCE of a circular section /, the MODULUS OP KUPTURE, has been found experimentally to have an average value of 7 800 Ib. per square inch for RED FIR. . Equating BENDING MOMENT to MOMENT OE RESISTANCE and allowing a Factor of Safety of 3-5 as required by the E.G. Regulations we have TlD 3 7800 ^prr, D (H -f A) 2 8 p 32 ' 3-5 + 2- 144 ' : Whence P = 219^ _ I ^ . (H + A}\ ti dO XI All dimensions in inches. Prepared from this formula, Fig. 49 gives values for the maximum permissible loading for poles from 6 to 14 inches in diameter and of ;'i 96 OVERHEAD POWER LINES lengths from 28 to 45 feet, assuming A = 2 feet. When P and // are known, the theoretically correct size of pole can be determined from this figure and the nearest standard, size then selected from Table XII. <o SIMPLE WOOD SUPPORTING POLES 1 irf^ 1 j 1 [ f O O 10 O 10 O" 10 l2 [2 o IO IO (=3 ' ' ' ' 1^ OO O3 O C<l ^ 1 O5 N CO rH 1O O IO H i I r-l I-H r-l i I M (M CO CO -<il -rP a! |||!ii ' 1 f III! "J** 1 H^H'M J-^ l^t<r<I<N-lHl MjTHW)^ j 1 J I I 1 o i i 1 1 i <M oo co t^ ia o i i~- oo o j tiD psSS i 3 Cf> P ^ 5 2 ,H , * **** H. ***. 1 & M -? rd f? = . "S p P .5"" 1 'S-'P -P p "o fl 1 "a <" CO O <N rh CO 00 O CM ^ CO O >O O 10 O IO O JO O IO .H a? r < C^l C<| C^ C^l C4 CO CO CO CO sj^ ^ IO &O CO CO I'* I"* OO OO i Q_j I 'ffl to Ill's "3 W 10 10 10 o P Sw ^ |a|s ' ' ' cb CO ij 1 * ib CO t CO CD Ol -^ t^ cb I ' PH m rM i-H I I r-H Ol t> n''? I ' 2>'o 1 i 1 oo co co co 05 as OS'OT o i-i' C^CO'TH | [ | -4 prt,2 r-l r-l r-l i-H r-t i^ F w.fci S i a & ! H .|.g "JR 1 1 rt rrt 1 0** rth* J* IS 1 * 1 COI>t-I> L-F-l^-t-OOOO t ' 00*00*0 111 i-O r^ b t> g IH 5*^ i*. 3 -> 3 a 3 CS g Ol 3 .g- . ^S? HM B^ 1 1 io"o>oco cocococo co'co' t- c~ i> 1 | | f Kj 03 .H3 s' \> ^ 1 If if i o f~H COCO-H^.O 10 co co i- i- oo oo g>j 4 g^s"- 10 1010 ln - ll A 4} 1 ^ f-n I-H c^i c<i c^i co cb ^ -^H ib cb i> cb ' 'I | ^ '0 in <y 1 4 H af--uii 3 -8 S q_< M fA i 60 co CD co co cs5 I- t- t^j^i^oo 00*05' | | ^ Ls ,is P a PH 3 S |l 1 u, .. H,^ ra ^c^, ' ^ lOlOlOlOCOCOCO CDCOCOCO CO t-" 1 'C P-I s a a s .1 .S S s Ci M I CU >o >o >o m 10 10 10 *o 10 10 10 >o to* III 1 ' rCt -H Ir.-g rt 01 KV 98 OVERHEAD POWER LINES It will be noted that the wind pressure on the pole arms, brackets and insulators is neglected. This is always relatively small and is largely compensated for by assuming the pole bo be. of uniform diameter, whereas it actually has a taper of about 1 in 100. The assumption that the pole would break at the ground level is close enough for practical purposes. This is where decay sets in, al- though it may not be theoretically the weakest section in a new pole. The standard poles are rated on their minimum diameter at a point 5 feet from the butt. The section considered above is at a distance from the butt equal to about one-sixth of the overall , length, i.e. for the lengths most commonly used, distances from 4-5 to 6-5 feet. As the taper is only 1 in 100, no' appreciable error is introduced. .. " A" the length of pole above the equivalent point of loading (i.e. the centre of pressure of the wind loads on all the wires) is frequently greater than the 2 feet assumed. The error introduced i by this difference is small for span lengths up to 350 feet or so with - the most common arrangements of conductors. Example. The wind pressure on a single 3 / -147 (-05 sq. in.) ice- covered conductor (f in. ice) = 0-710 Ib. per foot run. With four such conductors on a 250 feet span, the total horizontal lateral load on the pole due to wind pressure on the wires = 0-71 X 250 X 4 = 710 Ib. This assumes that the diameter of the earth wire is equal to that of the conductors, which is near enough for practical I purposes. Note in Table I. that the wind loading does not vary more than > 45 % over the whole range of conductors given. In this case, if a T V galvanised steel earth wire were used, the wind load would be ; 157 Ib. only instead of 177-5 assumed in these calculations. The sag at 122 F. = 4-02 feet (Fig. 15, p. 30), therefore (as- ? : suming that the point of loading coincides with the point of attach- ment of the lowest conductor) the required height of pole to point of loading = 20 + 4 = 24 feet approximately. From Fig. 49, p. 96. we find that for a load of 710 Ib., at a height of 24 feet, the diameter of pole at ground, level must not be less than 10-2 inches. If the arrangement shown in Fig. 26, page 45, is adopted, one , conductor will be 9 inches above the pole top and the other two conductors 33 inches below, and the total height of the pole out of || the ground should be 20 + 4 -f 2-75 = 26-75 feet. With 5-5 feet SIMPLE WOOD SUPPORTING POLES buried in the ground (see Fig. 55, page 110) the OVERALL Li ?OLE required = 26-75 + 5-5 = 32-25 feet. From Table XII. , page 97, it will be seen that the .- tandard pole which realty meets the requirements is 34 J inches, but a 32 feet/ 11 inch pole can be utilised if (which .cacticable) the two lower conductors are raised 3 inches. ,-ourse, leaves no margin for contingencies, but it is quite if 250 feet is the maximum length of span. If 250 feet is tin nngth some longer poles will clearly be necessary. In a 1 hen ordering it is advisable to obtain a proportion of the ; -ngths next above (and a few below) that upon which tl tions are based to allow for variations in length of span anc . "ound. It may be noted in this connection that the safe ' ad of standard poles in the same series is approximately i Mid independent of the length up to about 40 feet. OF PRESSURE FIG. 50. This will be clear from the points plotted in Fig. 49, which ,;;w loading point 2 feet from the top of pole and the buriec ' a^shown in Fig. 55, page 110. Therefore, having decided i writable length of pole for level ground it will not usually be ' y.-wy to repeat the calculations for longer or shorter poles wh: ; -'io, doubt be required in some parts of the line (unless, of cou] loading per foot run is increased in any way such as, for ex *U double- wired road crossings). The use of standard sizes of wood pole is presumed throi l!.us book, but it may often be possible to obtain non-standar nf economical prices. Using 32 feet/ 11 inch poles the distances of the various ; '' 1(> ffi the ground will be as shown in Fig. 50. Check The total BENDING- MOMENT on pole at the groui wiU be : 100 OVERHEAD POWER LINES CAP WIRE JJ26-5 X 12) + 9/x 177-5 = 58 000 ABM WIRES 24 x 12 x 355 = 102 200 EARTH WIRE (24 X 12) 28 x 177-5 = 46 200 J 10 s ^ / POLE ITSELF 26-5 x ^ X 8 x 13-25 x 12 =-. 28 100 Total B.M. in inch units 234 500 Therefore MINIMUM DIAMETER OF POLE required at ground line for a Factor of Safety of 3-5 s /234500x 32 x 3-5 1A0 . , = V 7TX7800 = 10 ' 2 mclies ' The 32 feet pole selected has a diameter of 11 -inches at a point 5 feet from the butt. The ground line is 5-5 feet from the butt, therefore the diameter at ground line equals 11 ' X = 10-94 * A.\J\J inches approximately and the pole is therefore of ample strength. We will assume in our calculations, however, that the diameter is 10*2 inches only. Centre of Pressure due to Wind Load on Wires.- Let x = distance of centre of pressure from pole top (Fig. 50), then wo have 177-5(3 + 9) = 355(30 - V) + 177-5(30 + 28 - x) Whence x = 27-25 inches. ^ Therefore the true height of pole to point of loading (i.e. the centre of pressure) = 26-5 ~?I|5 = 24-23 feet, instead of 24 feet as assumed. This means that a diameter slightly larger than 10-2 inches is necessary, but the difference is negligible. Use of Chart to Determine the most Economical Span, --The stress in a conductor is inversely proportional to the nag and if the proper sag is allowed, conductors of any size may be erected on practicaUy any length of span with equal safety. Prom a purely electrical point of view the longer the span the better, since tiiere are then fewer insulators, which are the weakest points in an installation. The modern tendency is to use long spans resulting in a reduction rn the number of supports and in simplifying the question of wayleaves. But it must not be forgotten that the SIMPLE WOOD SUPPORTING POLES 101 individual size, weight and cost of the supports themselves and of the insulators, arms, and brackets increases rapidly with span length because : (1) The sag is approximately proportional to the square of the span length. (2) The wind load is proportional to the span length. It will be clear, therefore, that there is always a particular span length for which, the product " Cost per Support X Number of Sup- ports " is a minimum. If a chart is prepared showing (a) Sag of wires at 122 F. in first quadrant, (6) Wind load on wires in second quadrant, (c) Safe nett loading of poles in third quadrant, it becomes a simple matter to select the span length wliich gives theoretically the minimum overall cost of the supports. Fig. 51 has been prepared for this purpose for single circuit high voltage lines with earth wire but without auxiliary conductors, up to 400 feet span. A similar chart will sometimes be found useful for L.V. lines. For general use it is better to enlarge the chart considerably, and for greater accuracy the second quadrant can be left blank until the particular arrangement and number of conductors to be used is known. If compound poles are used Figs. 65, 66 and 67 on pages 132 and 133 are available for use in the third quadrant. To illustrate the use of the chart Table XIII. has been prepared for the 3/-147 (-05 sq. in.) H.V. distribution line in our example. Where so manyiFacTfSfs are involved it was necessary to make a number of simplifying assumptions, among which were : (1) Arrange- ment of conductors and earth wire as in Fig. 28 ; (2) vertical clear- ance between 'conductors on same side of pole, 1 foot for each 100 feet length of span ; (3) centre of wind loading on conductors at point of attachment of lowest conductor. In preparing the Table, values were tabulated for span lengths varying from 150 to 400 feet at 10 feet intervals, but the only span lengths selected for consideration are those which best fit the stand- ard sizes of poles. The cost figures given are naturally only ap- proximate and are for the supports only erected in fairly good weather. They may have to be increased by 20-30 % in average English winter weather. 102 OVERHEAD POWER LINES < ^ 1 s 1 &>3 o * s^ * ^ K 1 ij i f i i X 9 e3 -dug) au;i qqSre^pg jo 8 I!Itt ja <i ISO;} TOJ 1O O 1O O t- CO lr- 1O CO CO CO CO i Ir-Sr li IrHrHp Ir ( -.WL,.) OOCOCOO1OO1OO 1O ^H "^ CO CO 1O C^l Oi cq <M IM it /.S,ft*L 10 ic 10 in in 10 10 10 "SO[Q C joj 9[]j\[ jod ^803 i^ojj cq cp h- i^ o '^ITJSI ^^ S^I^c[ JO *0^[ <N <N r- 1 CS O r ( Ol 1O O " Ol CO 1* 1O CO COCONC'-lr IrHrHl ( , W JTO1W >O CjJCl, r ^ OS TH CS *4t ^ I** pnu Suts,sajQ[ 'SJO^BI ID ^ CO CO CD t- 00 O O CO -nsnji 'i[JOAiuoij aio<i (ailS no uowuq -iJjsjf f s"u[pn[Dui) ooijj; >0 C^ji-HlOCOI"-! 1 THOl"^ r-(I P3l-il O ' 1 I-H 1 1 d CO gqcocoSscoco^i? opj jo t[q3uoi IIBJOAQ en co o 10 o 10 t co co r- i i co 10 t i t co cb I-H \j< 10 cb CM pnuoio o\.$ jo ;pio C33 CO IO O O CO CO T^ CO CO ' O C* 1 ! 1O ^l s ! C^ C 1 ! GN1 CO CO CO ' T -- ; - O IO O O O -JV jo qttio,! jo aouu^siQ ^(MWtMrocOCO-H TIIO.TJ) '[OAa'j PUUO.TF) xcs 10 o 10 10 10 ire CNIt-r lOl^BMOSCD ^ aioj jo K^UTBIQ i 1 t 1 rH i I r- 1 SaipTOi jo O2 CO o rH 1> (N C<) >0 O CO Viioao^ioajowSiaH CN 5S1 (N <N 0<| 01 g<| jo oonnqopanoiD oooooopo ' ( '* P C5 CO O O O O O JE o3Zl ^ US 1 t uudg jo iftStra'i JOt-os^cOi-iiog SIMPLE WOOD SUPPORTING POLES An average of Is. 6d. per annum per pole is taken for wayleaves. A very usual charge for single poles is 2s. 6d. per pole on arable land and Is. per pole on pasture. All other line costs, including con- 140 s 90 t Cosr ~F, \ ^ Fouo 7/a a .C Pec,* 5) 3 'jppo yra OF i-CAIf 193 C (19 3 .(. f?e< LLAt, *7 CklNDl/kTOK. C&'N ICE) SOO 250 300 310 LENGTH OF SPAN (Fecr) 400 JFia. 52.. Cost of supports for high voltage overhead line. H.D. copper conductors [-05 sq. in.], Single wood poles. ductors, stays, reinforcements at crossings, transport, supervision, etc., are assumed to be independent of length of span. Bird guards are not allowed for. 104 OVERHEAD POWER LINES The cost per mile is plotted against length of span in Fig. 5 from which, it will be seen that the minimum cost per mile occn with a span of 350 feet, but the variation in cost from 280 to 4( feet is only about 5 %. This result must be used with discretion. Assuming 131 fc supports and 150 for conductors and earth wire it might be suj posed at first sight that the line could be constructed for 300 or s per mile. But a stretch of several miles of simple straight line ; seldom practicable in this country and the reinforcements required a angles, terminals and crossings add appreciably to the overall cos of the line. The contract price for the line under consideratioi would probably be about 500, to allow for overhead charges an< profit, and therefore our saving of 5 % on the cost of the support is reduced to less than 2 % of the overall cost of the line. It i comforting to know, however, that unless we go to extremes 'the lengti of the span has little effect upon 'the overall economy. The cost of the supports under the 1923 E.G. Regulations (|- in ice) is also plotted in the figure and it will be seen that for the con- ditions assumed the reduction of the hypothetical ice loading tc f- inch has effected a saving of about 15 %. In order to draw attention to a point which is sometimes missed, the costs of supports for -193 conductors is also shown. It will be noticed that the supports for the smaller conductor are the more expensive. The reason is that, whereas the wind pressure on 3 / -14-7 is only 10 % greater than on -193, the latter has to be allowed 50 % more sag than the former. The difference in this case is only 12 per mile, but in some cases it is larger and it may well be the deciding factor when in doubt as to the advisability of installing a conductor somewhat larger than absolutely necessary from purely electrical considerations. Further Notes on Selection of Span Length. It will be noted in Table I., page 4, that on H.V. lines, the wind loading on the largest conductor (7 / -193) is only about 1 times that on the smallest (-162), but that the sag required with the smallest conductor is 4 times that required with the largest. Therefore, the larger the conductor, the shorter the pole required for a given span length and the longer the span possible for a given sag. As a method of making a tentative choice of span length up to 350 feet, or so using standard single wood poles, it is suggested that the length of span might be based on a sag of about 4 feet in still SIMPLE WOOD SUPPORTING POLES 105 air at 62 F. for all sizes of conductor. This may not give the theoretically most economical span, but all things considered; the values so determined give good results. Table XIV. 3 page 106, is given to illustrate the principles in- volved. In preparing the table, the span lengths to give 4 feet sag at 62 F. were first selected from Fig. 17 and then altered slightly where necessary so as to make the best use of the nearest size of standard pole. It will be noted that in some cases it is the length of pole which is the limiting factor and in others it is the butt diameter. In the former cases it will often be practicable to add a few feet by means of ironwork so as to increase the conductor clearance from ground, and in the latter it may be economical to use a pole longer than necessary to get the requisite butt diameter and to cut off a few feet from the top. Foundations of Single Poles. It is laid down in the Elec- tricity Commissioners' Overhead Line Regulations that the supports must be able to withstand the specified maximum hypothetical load- ing conditions without movement in the ground. This -requirement is reasonable with most forms of compound wood or iron structures, since any appreciable movement may so alter the distribution of stresses as to seriously weaken the structures. But in certain circum- stances a little movement may be most desirable in the case of simple wood or tubular iron poles, particularly if failure of the pole itself is thereby avoided. Moreover, the maximum ground reaction does not become effective until the soil packs up a little due to a small move- ment of the pole. Now, the properties of metals are well known, and we can pre- dict with reasonable accuracy the behaviour of good timber under stress, but when we have to deal with soil we can only guess within wide limits. With regard to the foundations of simple poles it is established that the pole tends to pivot about some point (Fig. 53 (a)) below ground, level, but the exact location of this point is somewhat uncertain. If the pole is assumed to be absolutely rigid the horizontal dis- placement (d) at any point will be proportional to its distance from this fulcrum [shown exaggerated in Fig. 53 (a}]. If it be further assumed that the soil has a definite elastic modulus, which is inversely proportional to the depth, it can be shown that the 106 OVERHEAD POWER LINES I a^iunj5 Md s2utAV JIUH ip o cb O CO 1 ib CO ip ib CO IO rH IO O H O do nv.ims -w <M S ^1 r-H ^^ CO CO o 00 cb o cp rH CO o ~? CO C33 O O3 IN IN CO rH (put AY qi ST) '& o&9 tpraui ^_3 O3 -4l o ib CO IO ip cb o cb O CO rH 03 CO CO p"!A\. - qi si 'HIM. 'i)T S9 '} t! St!<,' oiibi[qO | g r^i rH CO ib CO o ib IO ib O 10 (N 10 en ib l- lp CO IO rH ib '(PWIAY 'qi SI) notion " D tl jo opfuy jo 01113 i i CO CO o C3 cp 00 -H op 00 00 t~ CO (N 1 CO IN C3 CO CO CD cp flrtuo'SSKl'S^aT O3 cS CO CO CO IO IN IO CO rH CO XH % rH WSpAV 3 00 C3 CO CD CO (N rH o rH O * TptlO JO qil y I.Al ptJOd , cs 5 ^ M rH r-H 00 (N O cb >o O CO CO rH O IN rH IN o CO (M (N CO -HQQ iptjo uo PTK>I puiAV si IN s cp rH r 1 r-H CO IO r-H s 00 (N IO IO IN (N rH <N OS oo (N fce O5 rW rH 3 r-H rH N CO CO r-H ozjij 'pmpiruqg qsajiio'jsr (N CO 55 5S CO CO CO CO 3 rH % IO rH pojinDoy ai<T JO T HSJoT HIUOAQ +J <N CO o rH rH CO IO oo rH co ib CO oo CO rH co 10 CO O rH rH CO CO CO JCXpnpnOQ jo qujoj; oAoqti (HiSjoH |2' C33 cb 10 ip cb IO cb o cb O IO rH IO cb ! (OTT '<! 'QS Stji) puuoJD uj iftclaci; "S o (i^ ib 10 cp o <ji ib IO 03 ib O r-H CO IO rH CO o rH CO IO rH CO '(TOT '(I '19 -aivit 'qwip oos) onii pmiojo B *? 10 6 rH O r-H o CO rH o ip rH r-H o cp r-H IO 1O rH rH OJ CM rH snipwi s O 00 O <N o IO 00 fO CO O CD IO (N N O jo W o<t o^ wftoH ^S5 IO o5 IO C$ $ CD $ 1Q <N '(08 '<* '91 *3!Jl) -HV -w (M s =? o oo ' o 00 s S IO 01 O3 10 10 10 rH rH CO rH IO .ma^^s 'S O SI o FH <N o IO o (N IO r-H co IO CO CO g CO o rH IO CO CO rH ,o,, M ,- ew CO rH 0>0 CO <N O5 O 10 cp rH rH IO r-H o CM g CO O3 rH 00 CO co CD CD CD CD co O3 CO O3 joq.anpno(3 jo szig ^ ^ ^ -i -i ^ co CO i-^ I t- L- t> SIMPLE WOOD SUPPORTING POLES 107 ground reaction stress diagram is parabolic in form, as shown in . Fig. 53 (b), the fulcrum bein, of resistance of the ground, / h . Fig. 53 (6), the fulcrum being ~ from ground level and the moment 12 Jc.D.h* 12 . . ID. -ins. D is the mean diameter of pole below ground level in inches, \ the depth in feet, f b the maximum rupture intensity of stress of the soil in Ib. per square foot, k the maximum rupture intensity in Ib. per square fool per foot of depth, and M Q the MOMENT OF RESISTANCE oE ground in Ib. ins. If, on the other hand, we neglect elasticity (which requires a (ct) PIG. 53. stretch of the imagination when dealing with soil) and assume a maximum rupture intensity of stress, which is directly proportional to the depth, then the stress diagram is of straight line form, as shown 4 in Fig. 53 (c), the fulcrum ~= below ground and the MOMENT OF Vfr RESISTANCE of ground, _ / . D . h 2 _Jc.D.h 3 10 10 Ib.-ins. It will be noted that for a given value of Jc, the value of h required by the parabolic formula is only about 6 % greater than by the straight line formula, and so for practical purposes the difference is not very serious. It is generally thought that the parabolic formula is more correct in the initial stages of the loading, but when the 108 OVERHEAD POWER LINES foundations are on the point of giving way, the straight line formula is probably more accurate, and it is therefore proposed to make use of the latter formula in this book. The following experiment illus- trates its application. A picket 5 feefc long, 3 inches in diameter, 3 feet in ground (loam) began to give at 1 570 Ib. pull at ground level at right angles (Fig. 54). 3 in 10 X 1 570 X -/TjX 12 .-. Jfe = ^ = * = 4930. D . h s 3 x 3 3 A similar calculation gives a value of lc, == 5 920 by the parabolic formula. In this case the picket was driven in, and the conditions were rather more favourable than in the case of a pole fixed in a hole by Fro. 54. means of the earth thrown up in its excavation. If, however, the ramming of the refilled earth is well done Je should eventually rise again to the value it had in the virgin soil. It will be appreciated that precise values cannot be given to Jc for all the many different kinds of soil. Values up to 8 000 have been obtained in good gravel soil, but for made ground it may be less than 2 000. A conservative value for average good soil is 4 000. With, regard to a Factor of Safety, the 1923 B.C. Regulations specified 2-5 for the foundations but did not suggest any figures for maximum rupture intensity of the soil. The 1928 Regulations are more vague on this point, but in the explanatory memorandum issued with the Regulations it is implied that the foundations must be as strong as the pole. It is proposed in this book to assume values for the maximum rupture intensity of the soil and to allow a Factor of SIMPLE WOOD SUPPORTING POLES 10< Safety of 2-5 based on the specified weather loading and not on the strength of the support itself. It is believed that the foundation calculations in this and sub- sequent chapters will give sufficiently high factors of safety to satisfy the Commissioners. Buried Depth Required. Single Wood Poles. The SAFE MOMENT OF RESISTANCE due to ground reaction, neglecting the small difference between the diameter at ground level and the mean diameter below ground, and allowing a Factor of Safety of 2-5, D 10 X 2-5 Z) being the pole diameter at ground level in inches, h the depth in feet and k the maximum rupture intensity in Ib. per square foot per foot of depth. The SAFE MOMENT OF RESISTANCE of the pole at ground level, allowing a Factor of Safety of 3-5, 800 M G == *;: C" = -04& .D.h* Ib.-ins. Mp ~ 32 X 3-5 But the BENDING MOMENT referred to the fulcrum ~ feet below ground level will be found to be some 10 to 20 % greater than this value. Assuming the bending moment to be 15 % greater and equating one to the other, we get 04/c . D . 7^ 3 = 1-16 X 219D 3 , ,, 6300D 2 A ii - L . . h _ ______ If k = 4 000 Ib. / sq. foot, Ji 3 1-57D 2 . * For pole diameters from -7 inches to 13 inches the formula h = 04D -f- 14 is quite near enough for practical purposes. Curves connecting h and D are given in Fig. 55 for values of k = 4 000 and 2 000. From the lower curve we find that the pole selected in our example should be buried 5-5 feet, and we will now check this value. The BENDING MOMENT on the pole at maximum working load referred {*(* to a fulcrum ^ = 46-5 inches below ground level = 234 500 + 46-5(177-5 + 355 + 177-5 + 170-5) = 234 500 + 41 200 = 275 700 Ib.-ins. 110 OVERHEAD POWER LINES Now the MOMENT OP RESISTANCE OF GEOUND when the pole is buried 5-5 feet, assuming Jc = 4 000 Ib. / sq. foot Jf. . JDJi 3 4 000 x 10-39 x 66 3 = 691 500 Ib.-ins. 10 10 X 12 3 .. FACTOR OF SAFETY against overturning 691 500 275 700 = 2-51. J / g / / k- 200C > / 7 / / -to Si / / / / jj 56 / / / IS- Q / / ,x ^ "ff4 000 <=> g v / / 4 / A 78 9 10 11 '12 13 H 15 Dia.. or Pole, at Ground Level in Inches PIG. 55.' Buried depth of single wood poles. The F. of S. is actually higher than this as the pole selected has a much larger diameter than necessary. If, however, the soil is poor, and we take a value of Ik = 2 000 Ib. / sq. ft., the pole must be buried a depth of about 83 inches to be self-supporting with a F. of S. of 2-5. If buried 66 inches only, the ground can only provide a moment of resistance of SIMPLE WOOD SUPPORTING POLES m and therefore a moment of resistance of 275 700 X 2-5 345 750 ~- 343 600 Ib.-ins. must be provided by some form of fcmndatioii reinforcement. Assuming cross blocks of creosoted timber are used, one should be placed at a distance of one-third the depth and the other at the full depth, as shown in Fig. 56, as these positions are most effective in the initial stages of the movement. Let the blocks be 8 inches wide and the areas A-,, (lower) and A & 3?ia. 56. upper). If the stress diagram be drawn as in Eig. 56 it will be scon hat the average pressures over the blocks are without serious error 2 000 66 - 4 , . . , = 72 Ib. / sq. m. on A nd t 12 2 000 2? X ~ = 25-5 Ib. / sq. in. on A a . If each block provides half the required reinforcement, then w ave 3 taking moments about F, 72 X 15-5 x 25-5 X 24-5 X A z Whence ^|-~^J_54 and A 2 == 275 sq. ins. lowing for area of pole covered b^.iblocks, the total areas should bis 112 OVERHEAD POWER LINES Suitable lengths would therefore be 2 feet 6 inches for A and 4 feet for A z , but conforming with the usual practice A 2 might be made 5 feet, i.e. twice as long as A : . A thickness of 4 inches would be ample. (As a compromise, the pole may be buried about 76 inches and the lower foundation block dispensed with. The amount of excavation required will then be less.) Size of Bolts for Cross Blocks. The loads taken by the blocks a're approximately as follows (Fig. 56). A z (Upper) 171 A l (Lower] ~- = TOGO IK = 11 100 Ib. Taking/^ for wrought iron as 65 000 Ib. / >sq. in., assuming 3 000 Ib. initial tension in bolt due to tightening up, and allowing a F. of S. of 2-5, we have for TT 737 7 D 7, 7 Upper Block Bolt d r r?7 7 T> n 7 Lower Block Bolt d 10 OOP X 4 TT . 65 000 JTlOOx 4 TT . 65 000 A . ' = -44 in. -52 in. d being the diameter at bottom of threads, it will be advisable to use f-inch bolts in both cases. Two large washers should be used with each of these bolts. Shear Stress in Pole above Ground.It will be found that the shear stress above ground level is quite negligible. Below Ground. Load on Upper Block = 7 000 Ib. Load represented by shaded area above the fulcrum F in Fig. 56 54 = -z- X 46-5 X 10-22 = 12 800 Ib. A But these figures provide for a F. of S. of 2-5 in the foundation. Therefore the maximum Shearing Force at F at maximum working SIMPLE WOOD SUPPORTING POLES 113 the mean intensity. The diameter of the pole at F will be 1046 inch approximately, therefore, M r * v f co, o, 7 920 X 4 X 4 Maximum Intensity of Shear /Stress =-- - TTvJra - 5 = 123 Ib. / sq. in. Now the ultimate intensity of shear stress for red fir is about 4 000 Ib. / sq. in. across the grain and 500-600 Ib. / sq. in. along the grain. There is, therefore, nothing to fear across the grain, but since the intensity of shear stress is equal on planes at right angles there is a F. of S. of about 4 only along the grain at the point F. However, the shear stress falls away from 123 Ib. / sq. in. at F to 15-0 Ib. / sq. in. at the ground line. Normally, the shear stress is not a limiting factor with single poles but it is interesting to note that when single polos are tested to destruction the fracture frequently shows signs of longitudinal failure due to shear. Shearing forces become of great importance in compound poles of the "Butter " type (p. 116). Deflection of Single Poles. It can be shown that, for loads less than the elastic limit, the deflection of a cantilever of uniform cross-section at the point of loading = P . H 3 . 8 = ' inches. 3EJ P - load in pounds, H the length to point of loading in inches, E the modulus of elasticity in Ib. / sq. in and ,7 the moment of inertia. Unfortunately E is not very accurately known for timber but from experiments on fir poles it appears to have an average value of about 1-2 X 10 6 Ib. / sq. in. J for a circular section, about a diameter = ~, and P for a F. of S. of 3-5 = 219 . ^, 219 X 64 X _ 3 . E . J ~ 3 x 1-2 X 10 6 "x^ F72T 4 ' -00124 X ^-inches. 8 114 OVERHEAD i POWER LINES In our example, H = about 24-23 feet and D = 10-0 inches, therefore the deflection at the point of loading at the maximum working load 94. Q2 y 1 92 = 8 = -00124 X 10 = 10-5 inc kes. This is an elastic deflection, and the pole recovers when the. loading is removed. In the above the taper of the pole is neglected as is also wind pressure on the pole itself, but owing to uncertainty as to the value of E it is useless to pursue the matter further. It is important to realise, however, that the poles are flexible, and that the flexibility is an advantage. The lateral deflection" introduces a component of the longitudinal tension in the wires to help the pole laterally, and the longitudinal deflection which occurs when one or more wires break results in a reduction in longitudinal tension in the sound span and so tends to save the pole from breaking. 115 CHAPTER VII. COMPOUND WOOD POLES. UNFORTUNATELY, for ordinary heights and loads, large portions of trees have to be wasted. To reduce the waste to a minimum, standard poles should be used, but it will generally be found un- economical to use single poles for lateral loadings much exceeding 1 000 pounds. For ordinary H.V. lines carrying 4 wires this means a limit of span length of about 350 feet, but near the border line there will be cases where it is financially sound to stick to single poles, sacrificing a portion of the top in order to get a larger butt. If auxiliary conductors are installed it will invariably be necessary to use compound poles. There' appears to be a general impression that the overall cost of a transmission line is less if compound poles are used. This was probably true in many cases before the revision of the E.C. Kegula- tions in 1923, but it is certainly not true to-day, at any rate for single circuit 3-phase lines. Compound poles require a good deal more excavation work than single poles, and with the exception of the " Butter " type they take up a good deal more ground space. The only justification for compound poles in distribution work is the reduction in the number of wayleaves required. This minimises negotiation troubles, but there is no financial saving, since the rental for compound poles is about double that for single poles. Twin Poles. Consider two poles bolted together side by side, as in Fig. 57. T , 11 T , Am ^ 4 , ('K& d 2 \ 57rt 4 J VVl for each pole = / a a l + AS* = gj- + ( - X ^ J = -^-. y r 11 >7rtZ 4 1 57rt 3 ' Z ^ for eadl p le = -6T X T = ir 116 OVERHEAD POWER LINES K 3 *\ K 3 "\ and Z VVj _ for the twin pole = -|j- X 2 = -^-. 57rtZ 3 /.Z aai TT^ 1' 32 That is, the moment of resistance to bending in the plane of the bolt of two poles arranged as in Eig. 57 is FIVE times the moment of resistance of a single pole. This is interesting but purely a theoretical result. In practice it is not attained owing to the large shearing forces which are called into play and the bolts tear into the timber. Rutter Poles (Fig. 58). This design of compound pole makes use of the above principle but provides for the shearing forces by a series of hard- wood scarf blocks set into accurately cut slots. In this way the theoretical strength is realised ; in fact, as usually constructed with 4 inches between poles at ground line and 1 inch slots the Rutter pole is about 8 times as strong as one of its members used singly. That is the ratio of lateral to longitudinal strength is 4 to 1, which is the limit imposed by the E.C. Regulations. The shear blocks function as the web of a girder and are placed closer together in the foundation portion, because in that portion the shearing force is many times greater than in the part above ground (see p. 150). The resistance to overturning is provided by two long timber foundation blocks, placed in the plane of the wires, and secured to the poles by bolts. It will be clear that careful fitting is necessary, and as the number of slots is large they should be cut before creosbt- ing. The construction is therefore essentially a factory job. This type of pole is supplied by Messrs. Gabriel, Wade & English, Ltd., of Hull, and is delivered assembled, i.e. with the shear blocks and bolts in position, which obviates reassembling on site. The Rutter pole has a better appearance and is stronger, size " for size, than any type yet designed. ^ A " Poles. This type of pole is still largely used in this country for heavy lines. Let P (Eig. 59) be the'horizontal loading COMPOUND WOOD POLES 117 34 -Q - 3/iear Blacks jz "*6"*6" ~ Wes 9>te af Ground M O& Was/iera - Shear B/oc/c 6"t6'*s'.3" . Tbfi/bunc/at/'onB/oc/r /i/// $/ee/>rr ro**S"i'S!a Shear B/ocff <9' eeper /a"xs"xf'6 oc/r /o"x6"*ef FIG. 58. Butter pole. at the apex, and and T the compressive and tensile loads on the two members. Resolving along and at right angles to the compression member we have 118 OVERHEAD POWER LINES O P ' j_ T sin oc = P cos. 2i If, as is usual, the spread is equal to -Jth of the height, and OC -~ = 3-| approximately cos oc 2 = ^=8^ nearly and smoc = T VP + 8-13P = 8-2P also. That is, the compressive load on one member is equal to the tensile load on the other, and about 8 times the horizontal loading due to wind pressure. For calculation purposes it is usual to assume that, with the orthodox construction shown in Fig. 60, the compression member is equivalent to a strut fixed at both ends for which Euler's formula for the BUCKLING- LOAD = ' ' : 2 64. L* ' FIG. 59. If J is taken for the mean diameter of the pole and L the distance between the point of loading and the brace block, we get a result which is roughly confirmed experimentally. For example, consider an " A " pole made up of two standard 36 /11| in. poles D m = 10 ins. and L = 388 ins. and 64 . 388 2 = 155 000 Ib. P = ^ = = 1890011 , O A O'Zi (This omits the weight of wire and pole ironwork and of the pole itself, which would add about 5 % to the compressive load.) COMPOUND WOOD POLES For a Single Pole 766 = 766 n ' 53 P A 18 900 119 ' ' PS 3 470 G.I, Pole Roof = 5-45. I 4woshcr. I'Dia: Tuba Distance ple.ce.- irn JLL.,*.! L \^-3 -0" -H ; FIG. 60 (a). FIG. 60 (6). 40 feet creosoted red fir " A " pole. 10 inches diameter at groundline. 6 - 75 inches diameter at top. If these calculations are repeated for the 40 feet / 10 inch " A " pole illustrated in Fig. 60 it will be found that P A = 7 750, P s = 2 030 and P 7 7^0 . * A- ' '" v/ Q.QO ' P7"2030~ B ' 120 OVERHEAD POWER LINES It cannot be pretended that there is anything very precise about the above figures. No two poles are exactly alike and the timber itself varies. The effect of the lower scarf bolt, the tie rod and the earth reaction in reducing the free length of the compression member has been neglected. Moreover, a good deal depends upon the fitting which must be well done if the full benefit of the design is to be real- ised in practice. Poles constructed with a spread of one-eighth of the length have been found experimentally (see paper by C. Wade, JJ.JE.JE., August, 1907) to be approximately^ times as strong as one of the members used singly, and in practice " A " poles are rated on this value. The following instructions may be helpful in cases where it is decided to construct " A " poles on the job. " A " Pole Construction (see Eig. 60). Poles should be ap- proximately of the same dimensions and as straight as possible. If not quite straight, 'curvature should be at right angles to " A "' plane. 1. Scarf the poles. Cut ends of! square. Arrange the poles close together side by side with the tips supported on trestles. Twist the poles into such a position that the proposed scarves will be vertical and parallel to one another. Secure the poles tempor- arily in this position and draw parallel chords on the tips at distances equal to one third of the tip diameter from the point of contact. Then from the tip on the inside of the poles, mark off a distance equal to six times the tip diameter and draw chalk lines on each pole from these marks to both ends of the chords drawn on the tips. Remove the tapered portions with hand saw. "When the scarved surfaces are laid together, the poles should form an isosceles triangle with the butts at a distance apart approxi- mately equal to one-eighth of the height. If the distance apart differs much from this value, the scarved surfaces must be planed up until the correct ratio of height to base is obtained. 2. Fit the two scarf tie bolts. One bolt* to be 1 foot and the other 3 to 4 feet (depending upon length of scarf) from tip. The poles should be temporarily secured together at the tip with carpenter's cramp and at the butt with rough timber and nails before commencing to drill the bolt holes. Use -| inch self-clearing auger. Make sure that holes are horizontal and pass accurately through centres of poles. 3 inch by 2 inch by | inch washers to be used. The man using the anger can only check the direction in the horizontal plane. Another man should look after the elevation. COMPOUND WOOD POLES 13,1 3. Fix brace blocks. The brace blocks should be a driving fit in the slots. The slots should not extend into the heart wood of the poles. Brace block bolts to be 20 inches from butts. 4. Fit scarf block. Mark position of scarf block, which should be of oak 6 inch by 3 inch section and of a length equal to the diameter of the pole. The lower scarf bolt passes through the centre of scarf block. A mortice 1 inch deep is first made on each side with poles bolted together. Then separate the poles, finish slots with saw and chisel and complete fitting the block. 5. Tar all cut surfaces. Give all mortices, slots and bolt holes a coat of hot creosote-tar mixture. Then rebolt up finally. 6. Fit roof. To be fitted transversely to the line of wires. Top of pole beneath roof to be painted with creosote-tar mixture. 7. Fix tie rod. Tie rod to be fixed at a distance from the butt equal to about half the height of the poles out of the ground. Take same precautions as in 2, but greater care is necessary in this case as the holes in the two poles must lie in the same horizontal plane. Distance piece of 1 inch G.I. pipe to be fitted over bolt between poles. Four 3 inch by 2 inch by inch washers to be used. It will be found with the construction described above., that although the structure itself is 4-| times as strong as a single pole, if held rigidly at the base, the holding power of the ordinary type of foundations, for the same depth, is not increased in anything like the same ratio. To ensure having the strength of the foundations comparable with the rest of the structure, it will be necessary to add kicking blocks, and reinforce the connection between the poles and' the brace blocks. If the foundations give appreciably, the stress distribution in the poles is considerably altered. This remark applies more particularly to poles constructed with a splay less than one-eighth the height, in which case the poles pull over and fail by simple bending. See paper by Mr. W. B. Woodhouse, in J.I.E.E., February, 1929. It- is not the usual practice to attempt theoretical calculations for the design of " A " poles, but to base the construction on a pro- cess of trial and error. It must be admitted that the factors involved are so many, and the various conditions so uncertain, that calcula- tions unsupported by practical tests are of little use. Nevertheless, an attempt will now be made to bring out the salient features affect- ing the strength of the scarf joint and the holding power of the foundations. OVERHEAD POWER LINES " A " Pole. Scarf Joint. Consider the 40 ft. / 10 in. pol< illustrated in Kg. 60 (a). The safe working load for this pole is about 2 500 Ib. (Fig. 66, p. 132) applied 2 feet from the top, and the direct load on each member with this horizontal load will be 2 50C X 8 = 20 000 Ib. approximately. Actually, of course, there will be a number of wires at various distances from the top, and the centre of pressure will generally be more than 2 feet from the top. Bui from the nature of the problem close accuracy cannot be claimed foi the calculations. First assume two f-incli bolts to be used alone. The bearing lengths of the upper and lower bolts are about 5 inches and 7 inches respectively in each pole. . CO (d). First consider the tipper bolt. If there were no flexure the joint would fail through crushing of the timber, the stress in which is roughly indicated by the shaded triangles in Fig. 60 (c). In, practice, however, the moment of resistance of the timber on the outside halves of the bolt assisted by the frictional force between washers and poles is greater than the moment of resistance of the bolt, and therefore when the joint fails the bolts bend from their centres, as shown exaggerated in Fig. 60 (d). Neglecting the small inclination with the vertical, if P = longitudinal load on each pole and/ c the maximum safe compressive stress in timber in the bolt hole (1 030 Ib. / sq. in., vide p. 77), each of the triangular loads X 965 - Ib. COMPOUND WOOD POLES == = direct uniformly distributed compressive load, not shown 5 X *7o in diagram). The C.G-. of this load is f X f = inches from centre of bolt, therefore its moment of resistance / p\ 5 = {1030-r]S-lb.-ina. 995 Ib.-ins. The moment of resistance of the bolt nD* f TT . 27 . 60 000 32 J ' 32 X 64 X 2-5 therefore the total MOMENT OP KESISTANCE = J2 X | x (l 030 - |) -f 995 lb.-ins.| The BENDING MOMENT = 5P ; equating one to the other we get 5-82P = 4 4-30 and P = 762 Ib. Working similarly the safe load on the lower bolt will be found to be 895 Ib., making a total of 762 -h 895 = 1 657 Ib. The absurdity of such a joint will be at once apparent, so we will proceed to consider the state of affairs when a 6 inch by 3 inch oak block is inserted in the scarf at the lower bolt. The diameter of pole where block is inserted will be about 7 inches. e 2 FIG. 60 (& 571, .-. d = 110 (Fig. 60 (e)). a Length of Chord, AB = 2 X 3-5 X sin - = 7 X -82 = 5-74 inches. Length of Arc, AGB = 7rXJ>X^ = 7rX 7^^ = 6-73 inches. fi Area of Sector, OACB = TT X R z X 360 110 = TT X 3-5 2 X ^ = H-8 sq. ins. OVERHEAD POWER LINES 5*74 Area of Triangle, OAB = 2 X -^- = 5-74 sq. ins. ... Area of Slot = 11-8 - 5-74 = 6-06 sq. ins. Distance of centre of area of sector from centre ___ 2 Eadius X length of chord = 3 X length of arc ' 2 X 3-5 X 5-74- , nn . , - 3 X 6-73 = X '" mCll6S - 2 Centre of triangular area = 2 X -5 == 1*33 inches from 0. o If centre of slot area is x inches from 0, we have Q-OQx + 5-74 x 1-33 = 11-8 X 1-99, whence x = 2-62 inches. Now, if the scarf block be assumed to take the whole of the load, the average intensity of pressure on the timber in the slot will be 20 000 OAA ,, . . . . TJ, , f o , , f 6 000 - = 3 300 Ib. / sq. in., giving a JB actor of Safety of ^-^. D'Oo o oOO = 1-82, which is insufficient. The maximum permissible load on the scarf block - X 6-06 = 10 400 Ib. o-o Required Minimum Depth of Scarf Block. We will assume that the load of 10 400 Ib. is uniformly distributed over the area of the slot. As the centre of area of the slot is 2-62 inches from 0, it must be 0-62 inch from AB, therefore : ,,y, the BENDING MOMENT on the block =, 10 400 X -62 =- 12 900 Ib.-ins. The horizontal reaction will be a maximum at the upper and lower edges, falling away to zero at the centre, i.e. the load diagram will be triangular in shape, as shown in Pig. 60 (b), page 119. The total permissible load on each side of the centre, if d is the depth of the block, JL 0\J\J JL ^-j Cv rftf^ii-i = -^g- X ^ X 5-74 X g = 615cZ Ib. o .-. MOMENT OF RESISTANCE = 6I5d x ^d = 410d 2 . o Equating the Bending moment to the Moment of resistance we get 410cZ 2 = 12 900, and d = 5-61 inches. COMPOUND WOOD POLES Frictional Forces. These may be estimated as follows : The tensile load on the lower scarf bolt will be about 3 000 lb., due to screwing up and on the upper bolt 3 000 lb. + 615$ = 3450 Ibs. due to the scarf block reaction. Assuming a coefficient of friction of 0-3 for wood on wood, the vertical reaction will be about 9 450 X -3 = 2 835 lb. This is somewhat indefinite, perhaps, but quite appreciable. Help Given by Cross-Arms. Assume a 4 inch by 2 inch channel section cross-arm to be fitted into 1^-inch slots as shown dotted in Fig. 60 (6), page 119. It will be clear that any tendency of the compression member to ride upwards on the tension member, due to failure of the scarf joint, will twist the arm counter-clockwise. The stress diagram in the timber at top and bottom of slots will be triangular, as shown. Assuming pole diameter to be 7 inches, the modulus of the slot section will be Z c = ~bd z = 4 X 1-5 x 2-87 2 = 3-29 inch units 15 15 (see p. 80). If the arm bolts are 9 inches apart, and P = the maximum permissible direct load on the slots, we have roughly 9P=2/ C Z C , D 2 X 6 000 X 3-29 ' P = 9X3-5 = In addition to this Moment of Resistance of the slots there is a clockwise Bending Moment due to the wind pressure on the con- ductors, if pin insulators are used (see p. 78). Assuming 600 lb. wind pressure on each of two conductors, and that the conductors are attached to points on the insulators 8 inches from centre of cross-arm, the bending moment due to this eccentric loading on the cross-arm = 600 X 8 X 2 == 9 600 Ib.-iiis/ and the vertical load on the slots = = 1 067 lb., of which -5-=- == 305 y o'D , lb. may be added to the vertical working load. Similarly, if a second arm is fitted higher up where the distance 9 between bolts is 6 inches, further additional loads of 1 250 X TT 6 9 = 1 875 lb., and 305 X r = 457 lb. can be dealt with. 126 OVERHEAD POWER LINES With, two arms and a scarf block, therefore, the TOTAL LOAD whic the pole top joint will support = 10 4-00 + 1 250 + 305 + 1 875 + 457 + 762 = 15 049 Ib. (7G2 Ib. is allowed for the upper scarf bolt, but the lower one i neglected, since, in addition to the tensile stress due to tightenin up, it has to withstand the reaction between the scarf block and th two members.) This is still a good deal less than 20 000 Ib., the value based upo] the assumption that the pole is 4-| times as strong as one of it members used singly, and to enable the full strength of the pole t< be developed a second scarf bloclc is really necessary if, as assumed the load is applied 2 feet from the top. In practice, however, as the centre of pressure would be mucl greater than 2 feet from the top, and friction has been neglected there is no doubt that the Factor of Safety would be adequate witl a load of 2 500 Ib. If a second scarf block is fitted it will be necessary to check tin shearing strength 6f the pole above the block. "A" Pole Foundations. In practice the foundations ar< found to fail by the pulling out of the tension member. It is there fore proposed to assume that the brace block bolt in the com pression member is a fixed point. The figures for ground stress will be taken from Fig. 91, page 181 The upward ground reaction will be assumed to be equal to tli< horizontal ground reaction at the same depth. It is actually greater but it is important to avoid any settlement. The horizontal loac of 2 500 Ib. applied at a point 2 feet from the top of the pole pro- duces a BINDING MOMENT of 2 500 x 8 x 60 = 1 200 000 Ib.-ins on the foundations, if we neglect the small inclination of the pole with the vertical. The upper surfaces of the brace blocks are 5 inches from ground level, at which depth the safe upward bearing pressure = 10-5 Ib. / sq. in. approx. The downward ground reaction will be triangular in shape, as shown in Fig. 61, and the total safe load -jr- X 84 X 10 = 4 400 Ib. 2i The C.GL of this load = 84 X f = 56 ins. from the brace block bolt, therefore the MOMENT or RESISTANCE = 4 4-00 x 56 = 246 000 Ib.-ins. To this may be added the MOMENT OF RESISTANCE of the com- COMPOUND WOOD POLES 127 pression member to lateral earth pressure. Since there is a large volume of disturbed soil and any appreciable movement is undesir- able, we will take k = 2 000 in the straight line formula given on page 107. 2 000 X 10-5 X 78 3 Then M 10 X 12 3 X 2-5 = 231 000 Ib.-ins. ,78 This implies that the compression member pivots about a point = = 55 inches below ground level instead of 58 inches as assumed otherwise in these calculations, but this makes no material dif- ference to the result. 78" 49" J. Mv /Two Brace Blocks. I0"5"i<9'lot1<j. Kicking Blocks 1 "''U'O'-elon^. / i D H 3' -6" FIG. 61. p, ,, We may also add the MOMENT OP RESISTANCE of the slots which, estimated as explained on page 79, will be about 4-3 000 Ib.-ins. ft There may further be a little help given by the upward reaction <Jn the brace blocks outside the compression member, but this will tjiie very small owing to the fact that the brace blocks share the large Downward load of 20 000 lb., which produces a crushing stress on the ;oil exceeding 30 lb. / sq. in. The maximum safe downward stress tjg ,!;3ijp. the soil at a depth of 63 inches, assumed equal to the safe horizon- :y' };al stress, is about 45 lb. / sq. in. '"' The total MOMENT OP RESISTANCE, then, which can be counted upon = 246 000 + 231 000 + 4-3 000 = 520 000 Ib.-ins., which is , quite inadequate. It may be noted here, that, prior to 1923 the B.C. F. of S. for 128 OVERHEAD POWER LINES wood poles was 10, and the maximum bending moment on these 3-5 foundations would have been 1 200 000 X -^r - = 420 000 Ib.-ins. only, to deal with which a simple single brace block would have been considered quite sufficient ; and in fact proved to be so in practice. It must be pointed out, however, that after a year or so, when the earth gets consolidated, the strength of the foundations is greatly increased and may in favourable circumstances reach double the initial value when the pole is erected. Now that the F. of S. for wood poles has been reduced to 3-5, it is obvious that more attention must be paid to the foundations, and we will now consider the effect of adding KICKING BLOCKS at the ends of the brace blocks. The detailed calculations giving the moment of resistance due to the upward ground reaction on the brace blocks and kicking blocks at the foot of the compression member will be left as an exercise for the reader. Together with the moment of resistance of the slots it will be found to be nearly equal to the MOMENT OF RESISTANCE of the brace blocks themselves which = H-f = 2X5X100X7800 = ^ 6 6x3-5 The moment of resistance to be provided by the kicking blocks at foot of tension member is therefore 1 200 000 - 246 000 372 000 - 231 000 = 351 000 Ib.-ins. If the kicking blocks are 4 inches thick their upper surfaces will be 49 inches from ground level. The safe upward ground stress at this depth is about 9 Ib. / sq. in., therefore the area of the kicking blocks must not be less than . , 351000 Tf 1 2 innhp.s wide, a length of 3 feet 6 inches will do. Load on Slots. Working as shown on page 123 ; the area of the 2|-inch slot is about 15-4 sq. ins., iner edge 8-66 inches, and the distance of C.G. of COMPOUND WOOD POLES 129 slot area from inner edge 1-02 inches. Neglecting the area of the bolt itself, the compressive stress- on timber in slots 20000 . = 15^-2 = 650 lb./s q . m. [neglecting the relief due to lateral ground reaction]. The safe working stress on the timber in compression 6000 , _ K1 , . _ - = 1 715 Ib. / sq. in. O'O and from this point of view, therefore, the slots need not be so deep. About 1-g- inches would be sufficient. Shear Stress. The stress tending to shear off the pole below the slots 650 X 154 The maximum safe longitudinal shear stress 500 _ . Q . = = 143 Ib. / sq. in. o'D There is ample security here also, and we could safely place the brace blocks several inches nearer to the butts. Twisting Moment on Brace Blocks. The above cal- culations for stresses in compression and shear are only true if there is no appreciable movement of the brace block in the slots. If we assume uniform stress distribution over the bottom of the slot, the C.G. of the slot reaction will be 1-02 inches from the inner edge, and taking the C.G. of the load on the brace block to be on its centre line there will be a TWISTING MOMENT on it = 10 000 (2-5 1-02) 14 800 Ib.-ins. (Fig. 62 (a)). Now the slot fixing is- quite incapable of dealing with. this. If the brace block twists ap- preciably the concentration of stress near the outer edge of the bottom of the slot will be so great as to crush the timber, the brace block will become loose and as the f -inch bolt has a safe moment of resistance of 995 Ib.-ins. only the joint will fail. It may be noted that the centre of the brace block' is not the best position for the bolt in the tension member. It will be much more effective if placed well above the centre (say 2|- inches above), but in this case a second bolt should be placed below the centre to satisfy the conditions when, owing to change of direction of wind, 9 130 OVERHEAD POWER LINES the member becomes loaded in compression. Moreover, if a narrower brace block is used and housed full depth, the twisting moment will be very much reduced, but reducing the size of the brace blocks will necessitate further modifications in the design. Fortunately, in the case under consideration, a considerable moment of resistance is provided by the fact that the kicking blocks are bolted to the brace blocks. The compressive load between brace blocks and kicking blocks is indicated by the small triangles in Pig. 62 (6), and the safe maximum MOMENT OF RESISTANCE 1500 5 5 = 6 430 X 1-67 X 2 = 21 400 Ib.-ins., which is greater than necessary. 10,000 pM -I-4&" I r rf" f T -~ jf 4" ik! -t. \ 10" J j, 5. i i \ i i i i i U-" 1 1 ^ F i i 4-" HI ~ i ... PIG. 62 (re) in slota. FIG. 62 (6) at kicking blocks. Brace block sections. The stress in the f-inch bolts securing the kicking blocks to the brace blocks allowing 3 000 Ib, tension due to tightening up the nut 3 000 + 6 430 X U 8 21400 304 = 24 500 Ib./ s<j. in. The maximum safe working stress = 26 000 Ib. / sq. in. a'O It is not proposed to pursue this matter further as it is thought that enough has been said to indicate the lines on which the strength of the foundations may be estimated. COMPOUND WOOD POLES 131 Wo have neglected the loss of area of brace blocks in slots and under kicking blocks and a number of other small details, but the method of calculations will bo found to give results which agree closely with those obtained by experiment on poles shortly after erection in reasonably good soil. Better results will generally bo obtained when, the soil gets thoroughly consolidated (say after six to twelve months). If __J L xf. J || J>-l |-n fy Q " -f] I'ru. 63. Butter fcyyo. METAL STRAPS, .Fxu. 04,--Aiu!hoi.'a typo. H of Hfcrongtihonod "A" jiolo fuuudal/itniH. A, good deal of tlvought has been given during the last two years to the- design of satisfactory "A" polo foundations and the two designs illawtrated in Figs. 03 and 04 aro of interest. Rutter " A " Pole Foundation. -Fig. 03, shows a new type of " A " pole foundation patented by Mr. Kutter. Braces are housed full depth in the poles, near the bottom, and the usual transverse kicking blocks aro provided. The braces are connected to the poles 132 OVERHEAD POWER LINES 8000 Wind Load on /ce Covered Wires in Lbs. 7000 6000 5000 4000 3000 2000 1000 \\ \ \ \ V\ X XA \u r\ \ 10 Diameter of Pole at Ground Line in Inches Fio. 65. Net safe working loads. Rutter poles. Wind Load on Ice Covered Wires in Lbs. 7000 6000 5000 WOO 3000 2000 1000 Diameter of Pole at Ground Line in Inches IG. 66. Net safe working loads. "A" poles. COMPOUND WOOD POLKS 133 Ly stirrup straps which are proportioned to the load both in sectional area and length, so that a sufficient number of bolts can be provided. Using narrower brace blocks housed full depth reduces the bending moment on the bolts, and incidentally the stirrup straps enable the brace blocks to be fixed nearer the butts. Anchora "A" Pole Foundation. Fig. 64 illustrates this type of " A " pole foundations, which has recently been patented. The special features of the design are (1 ) the wrought iron tie rods which transfer the upward pull from the kicking blocks to the tension member ; (2) the undercutting of the earth to take the kicking 4000 3500 Wind Lo&d on Ice Covered Wires in Lbs, 3000 2500 2000 1500 1000 Diameter of Pole at Ground Line in Inches Kid. 07, Nob Hiife working loads. "PI "polos. blocks, thus taking a proportion of the \ipward load by undisturbed soil ; (3) the brace blocks are situated in their most effective position, viz. at the pole butts (maximum depth) ; (4) the metal plates of channel .section fitted to improve the bearing surface under the butts. " H " Poles (Fig. 85, page 174). "H " poles, as usually con- structed with one set of trussing tackle, are not so strong laterally as " A " polos, but they arc simpler to construct. They are "largely employed, for terminations, junction and sec- tioning poles and for road and telegraph crossings where a greater ground clearance is required, owing to the longer arms which are 134 OVERHEAD POWER LINES possible and the extra space available for dealing with, conductors stay and guard wires and for fixing pole type transformers, isolating switches, cable terminating boxes, etc. Also it will frequently b< necessary to use them at angles and terminals in cases where th< direct compressive load is too much for a single pole. Eigs. 65, 66, and 67 give for BUTTER " A " and " H " poles respectively, the safe load that may be applied to a point 2 feel from the top of a pole, taking a modulus of rupture of 7 800 Ib. pei square inch and deducting the wind pressure calculated at 8 Ib. pe7 square foot, after dividing by a Factor of Safety of 3| as required by the E.C. Regulations. For the " A " and Rutter poles, the wind pressure is taken on 1-| times the projected area of one pole, and for the " H " pole If times. The figures for RUTTER poles are obtained by calculation, allow- ing for 'the weakening due to the 1-inch slots and one f-inch bolt. " A " poles are assumed to be 4-| times and " H " poles 3 times as strong as one of the members used singly. It is understood that these assumptions are 1 approved by the Electricity Commissioners. 135 CHAPTER VIII. , IRON AND FERRO-CONGRETE POLES. IRON POLES are not often used if wood poles are suitable and easily procurable. Por the same duty the former are much more expen- sive than the latter, and as iron poles have to be painted periodically they cost more in maintenance. In the colonies and India, however, climate and the white ant frequently make the use of iron poles imperative. In this country TUBULAR IRON poles are employed occasionally in residential dis- tricts for aesthetic reasons since they take up less room and lend themselves better to painting and decoration. The British Engineering Standards Association have recently issued a specification for TUBULAR IRON AND STEEL POLES for Telegraph and Telephone Purposes (B.S.S., 134r 1927) from which the following particulars have been abstracted by permission of the Association. Official copies of this specification may be obtained from the offices of the Association, 28 Victoria Street, London, S.W. 1, price 10s. 10d. 3 post free. A good many of the standard sizes are suitable for power distribution, including 37 feet poles for working loads up to 1 000 Ib. Four types are standardised, two giving lengths up to 44 feet in two or three parts, and the other two made up of short lengths not exceeding about 8 feet each. These latter " multiple section " poles are the most suitable type for use abroad, as their handling, storage and transport are simpler. " Type B " consists of a series of tapered steel riveted tubes, galvanised after manufacture, with a cast iron base so combined as to give the required strength and length. Any series of tubes car be nested together into a package 8 feet long, the diameter of whicl is equal to that of the largest tube. This method of packing avoids the necessity for any further packing and ensures economy in ship ment. 136 OVERHEAD POWER LINES To construct tlie pole, tlie bases and tubes are laid on the groun and the tubes so placed that the riveted seams appear on alternai 36.4 S'.B (a) PIG. 68. Riveted seam tapered pole in multiple sections. British standard Type B. Test load 1 100 pounds applied 6 ins. from top. sides. The lower tube is first driven on the base, and the seconc tube driven on to the lower one, and so on, until the pole is complete As an example, Fig. 68 (a) shows the assembly of a Type B stan IRON AND FERRO-CONCRETE POLES 137 clard pole of overall length, 36 feet 4 inches (nominal 35 feet) capable of withstanding a test load of 1 100 Ib. applied at a point 6 inches from the top, when buried in the ground 5 feet 6 inches. Fig. 68 (b) shows the joint between tubes F and E, Fig. 68 (c) the tube F, Fig. 68 (d) the cast iron base, and Fig. 68 (e) the buckled plate. The ultimate tensile strength of the steel is specified as 27-32 tons per square inch, and the elongation 20 % on an 8-inch test piece. The ultimate tensile stress of the C.I. used in the base must not be less than 9 tons / sq. in. The yield point stress in the steel tubes, calculated from specified cantilever test loads must not be less than 20 tons / sq. in., and the C.I. Base must withstand a test load producing a calculated stress of 9 tons / sq. in. without breaking. The test load for the assembled pole is based on the assumption that the ground does not give effective support for a distance (in the case of this particular size of pole) of 10 inches. For purposes of calculation, therefore, the " length above the ground " is taken as equal to 30 feet 10 inches + 10 inches = 31 feet 8 inches. The bending moment on the section 10 inches below ground level is therefore 1 100 X (380 6) = 411 400 Ib.-ins. when the test load is applied. As a matter of interest we will check the stress at two of the sections. Maximum Stress in Steel Tube F, just below top joint (at fitting line). From Fig. 68 (a) it will be seen that, at the section chosen, the approximate external and internal diameters of the tube are 9-0 and 8-744 inches respectively and the distance of the section from the point of application of the load = 261 inches.- ' D 4 d* The MOMENT OF RESISTANCE of section = TT -/ m which D and d are respectively the external and internal diameters and / the maximum stress. The BENDING MOMENT = 1 100 X 261 Ib.-ins. ; equating one to the other we get f _ *r(9*-8.7M') J ~ J 32D ~ 32 X 9 whence /= 37 000 Ib. / sq. in. = 16-5 tons / sq. in., which is well within the specified figure of 20 tons / sq. in. 138 OVERHEAD POWER LINES Stress in Cast Iron Base 10 inch below ground level- Beference to Fig. 68 (d) will show that the cast iron base has, a the point chosen, the approximate external and internal diametei of 8-325 and 7-325 inches respectively, and the distance of th section from the point of application of the test load = 374 inches 7r(8-325 4 -7-325 4 ) f , 1AA ot _. n . ... 32 x 8 . 325 ^ 1 100 X 374 lb.-ms. 5 whence / = 18100 Ib. /sq. in. = 8-1 tons/sq. in., which is we within the figure of 9 tons / sq. in. which the material has to with stand to comply with the specification. Assuming an ultimate stress of 30 tons per square inch, the maxi mum working load for this pole will be $ 660 Ib. applie< .Z'O a\J 380 6 inches from the top or 600 X ^^ = 694 Ib. applied 2 feet from th top. Allowing for wind pressure on the pole itself, it will carry a H.V line (f inch ice) consisting of three -05 s'q. in. copper conductors an< one 7/-08 earth wire with the centre of pressure 2 feet from top on i span length of 220 feet, but the wires will be nearly 6 feet highe than they need be. If the conductors are lowered 4 feet, the spai length may be 250 feet, and of course the top member " c " need no be more than 4 feet long. In tropical countries the pole should be suitable for longer spans The total weight of the pole is about 800 Ib. (tube C, 65 ; D, 92 E, 120 ; F, 154 ; cast iron base, 350 Ib. ; and the buckled plate 1* Ib.). The weight- of an equivalent wood pole would be about 900 Ib If weight is an important consideration, it may be noted tha- Type D poles for the same duty can be obtained in high tensil< steel (up to 40 tons / sq. inch), weighing 520 Ib. total, the maximun weight of any one part being 204 Ib. But since the steel tubes ar< buried in the ground, and no buckled plate is used, such poles shoulc be set in concrete. Foundations (see p. 105). / BENDING MOMENT referred to apoint = = 46-7 inches belov V2 ground level = 660(364 -f 46-7) 271 000 lb. : ins (Fig. 69). IRON AND FERRO-CONCRETE POLES D . Jc . Ji s MOMENT OP RESISTANCE OF GROUND = M = 139 (p. 107). It will be seen from Fig. 68 (d) that the average value of D below ground level is 8-6 inches. Assuming then that Jc 4 000, 8-6 X 4000 x 66 3 10 X 12 3 .. F. of S. against overturning = 571 000 Ib.-ins. 571 000 2-71 000 = 2-11. 660 LBS FIG. 69. This is insufficient, but it is usual, however, to add a buckle plate, 20 inches square fox the pole we are considering. From Fig. 91, page 181, we find that the maximum pressures which the ground will withstand at a depth of 5 feet 6 inches are : (1) Uplift at end A (Fig. 70) = 2 360 X 2-5 = 5 900 Ib. / sq. ft. (2) Downward at f end 5=7 080 X 2.5 = 17 700 Ib. / sq. ft. This assumes the downward pressure which the ground can support to be equal to the pressure offered to a stay block when pulled horizontally. Actually the former should be much greater than the latter. To these figures may be added the weight of the pole, wire, ar fittings, which will be about 1 000 Ib., giving a downward pressure X 144 = 360 Ib. / sq. ft. Therefore the maximum pr< .20 X ^0 sures when the ground is on the point of failure will be : 13-Slbs Fia. 70. 140 OVERHEAD POWER LINES (1) End A, 5 900 + 360 = 6 260 Ib. / sq. ft. = 43-5 Ib. / sq. in. (2) End B, 17 700 - 360 = 17 340 Ib. / sq. ft. = 120 Ib. / sq. in. The load diagrams on the upper and lower surfaces of the buckle plate will be as shown in Fig. 70, therefore the moment of resistance due to the plate = 109 000 Ib.-ins. This neglects the loss of area on the upper side of the plate due to the. pole, but this introduces no appreciable error. Total moment of resistance of ground will then be 571 000 + 109 000 = 680 000 Ib.-ins., and the F. of S. against overturning is increased to 680 000 r . , . 271 OQO = 2 ' 5 approximately. If the buried depth is increased from 66 to 70 inches the F. of S. goes up to about 3-0. Deflection. It is not possible to calculate this very closely, but we can make a guess at what to expect, in the following way : PH 3 In the case of a uniform tube the deflection 8 = -^-^^ and 3EJ if the tube were conical with the apex at the point of loading, JD//3 = . The actual deflection will be somewhere between these 2EJ two values. Assuming the deflection to commence at the top of the lower band in tube F, then H = 3.25 ..inches, external diameter of tube = 10-026 inches and internal diameter =9-77 inches approximately. T * / r _________ ! 7r(10-026 4 9-77 4 ) , -" i " ........... . ...... ..... *"..'.'"" 4-ri 64 64 Taking E = 30 . 10 6 Ib. / sq. in.,. 66QX325 3 _ ~Wj~ 3 X 30 X 10 X 48 - For a conical tube the deflection equals 15-7 X 1-5 = 23-5 inches. So a rough estimate of the deflection to be expected at the maximum working load is the mean of these two values, say, 19 inches. Channel Iron Poles. The tubular poles described above would naturally be used if available, but in some cases abroad it IRON AND FERRO-CONCRETE POLES 141 may be necessary to make up poles from sucli standard sections as may happen to be at hand. Channel Section is very suitable for the purpose. (a) (6) If ftr i fca^ DETAIL. / J 2."xs"x-fa AT'AV g- s* Washer (t / 4 jfe^^j| ? ' n 6" % SOLT5- Jfia. 71. Simple channel iron pole. Fig. 71 (a) shows a suggested design for a channel iron pole which is approximately equivalent in strength laterally to a 32 foot by 9 inch (i.e. medium) fir pole. The pole is made up of three lengths of standard channel, one 11 feet of 6 inches by 3 inches, one 11 feet of 5 inches by 2|- inches, 142 OVERHEAD POWER LINES and the other 12 feet of 4 inches by 2 inches. The lengths overlap one foot and are bolted together. A simple way of studying the strength of the pole is to draw a MOMENT OF RESISTANCE diagram (Fig. 71 (&)) from the figures given in Table VIII., page 71. It will be seen from this diagram that, assuming 5 feet G inches buried, and that the point of loading is 2 feet from the top, the pole is suitable for a BENDING MOMENT of about 147 000 Ib.-ins., with a F. of S. of 2-5. Allowing for wind pressure on pole itself, the lateral load which the pole will carry 14 7000 24-5 X 12 21 = 500 21 = 479 Ib. The pole is therefore suitable for a H.V. line (f-inch ice) with three -05 sq. in. conductors and a 7 / -08 earth wire on a span length of 175 feet. The total weight of the pole is about 440 Ib. and that of an equiva- lent wood pole 600 Ib. Ratio of Lateral to Longitudinal Strength. (See Table VIII., p. 71.) 7-09 6 ins. X 3 ins. Channel Section == ^ Q = 5-3 [4-57 only utilised], , .L'iJO >) 4-749 5 ins. x 2-| ins. ,, = -^g- = 5-0, 4 ins. X 2 ins. ,, = -^^ 5-05 [4-4 only utilised]. J The B.C. Regulations specify a maximum ratio of 4, therefore in the unlikely event of such a pole .being used in this country, the loading would have to be reduced from 500 Ib. to 500 X & 400 Ib. It will be noted from Fig. 71 (b) that the strength of the pole is determined by the strength of the 5 inch X 2| inch section. Strength of Foundations. Bending moment referred to fulcrum (Fig. 71 (a)) p. feet v 2 below ground level = 500 (24-5 X 12 \ = 147 000 + 23 300 = 170 300 Ib.-ins. IRON AND FERRO-CONCRETE POLES 143 MOMENT OF RESISTANCE of ground (p. 107). Dkh3 3 x 4 OOP X 663 10 x 12 a . 200 000 lb,ms. .-. F. of S. against overturning = 1 . = 1-17, which is insiifficient. X i o\j(j If a sole plate is attached to the foot of the pole, it will help matters, but the calculations on p. 139 show that we cannot hope for more than another 100 000 Ib.-ins. from a substantial 20 inch by 20 inch sole plate. Cross blocks will therefore be necessary, and these should preferably be of concrete slabs (seep. Ill for calculations). However, it is perhaps better to set such poles in concrete. The stability can then be easily ensured and, moreover, steel embedded in concrete lasts indefinitely. A concrete block 12 inches X 12 inches X 72 inches is suggested. The bearing surface width will then be increased to 12 inches and the depth to 72 inches. Now, although a slight movement in the ground is unobjection- able when simple wood and iron poles are planted direct in the ground, it is advisable to ensure as far as is reasonably possible that there shall be no movement whatever of a concrete foundation. This is of greater importance in the case of lattice girder structures than in the simple pole under consideration, but it is recommended that in all cases when calculating the moment of resistance of the lateral earth reaction to concrete pole foundations the value of k should be taken as 2 000 in good soil. The MOMENT OF RESISTANCE will then be 12 . 2 OOP . 72 3 MQAnnl1 . = _ = - 1Q 123 - = 518 000 Ib.-ins. and the F. of S. against overturning = 3-0. This neglects the small moment of resistance due to unsym- metrical earth pressure under the block. Concrete Pole Foundations. The concrete should be com- posed of clean gravel (or ballast) or hard broken brick or stone, with sharp clean sand mixed with Portland cement in sufficient 144 OVERHEAD POWER LINES proportions to fill the interstices of the coarse material. The mixtures usually employed are : 1 cement, 5 sand, 10 gravel or broken stone (graduated) ; or 1 cement, 4 sand, 6 broken stone (3 inch to 2 inch mesh). The gap space with gravel (pebbles all sizes from 2-|- inches to s ^ inch) is about 35 % and with broken stone if graduated to include the same sizes it is about the same. If the broken stone is larger (3 inch to 2 inch mesh) the gap space will be larger and it is therefore not so economical in cement. The weight of materials in Ib. per cubio foot may be taken as follows : Cement 90, sand 90, gravel 110, concrete 135. It must not be overlooked when estimating that the sum of the volumes of the constituents before mixing is some 40 to 50 % greater than the volume of the resulting concrete. A suitable natural mixture of gravel and sand can often be ob- tained on site. To make the concrete, the cement is first well mixed with the sand, dry, then water is added, mixing all the while until the con- sistency of moist earth is reached. The aggregate of broken stone or gravel is then added and the whole well mixed. The filling should be done in 6-inch layers which must be well rammed until a layer of water appears on the surface. If the work is well done a compressive stress of about 1 700 Ib. / sq. in. can be counted upon in 30 days after setting, and it is there- fore quite unnecessary to use richer mixtures. It is most important to see that no earth gets mixed with the concrete, as its strength may thereby be seriously weakened. The concrete should extend six inches or so above the ground and the top should be sloped ofr a little to prevent rain from settling. The top surface should be faced with a 1 : 2 cement mortar. Compound Channel Poles. For larger loads, compound channel poles may sometimes be found useful. A simple example will be considered, consisting of two of the channel iron poles de- scribed above arranged as in Figs. 72 and 73 (a) with 12 inches between backs of 6 inch by 3 inch channels. The information in the following Table XV. will be required in, addition to that given in Table VIII., page 71. IRON AND FERROCONCRETE POLES Y <=' 1 4,5 POUND -INCHES. FIG. 73. Compound channel iron pole. 10 146 OVERHEAD POWER LINES TABLE XV.[B.S. Spec. No. 61924.] Channel Section. Distance of C.G. from Back of Channel, ins. h, ins. , ins. 4X2 599 24 31 5 X 2 773 25 38 0x3 890 25 38 Strength of 6 inch x 3 inch Twin Channel. J Ml = J VVl + A.S. 2 (Fig. 72). Loss of area due to two |~inch brace bolts (or rivets) == -5 X '38 X 2 = -38 sq. in. .-. Reduction in J^ = -38 X -66 2 = -165 and Reduction in A.S.* = -38 X 4-5 2 = 7-69, both values being approximate only. .-. Nett J COi = (2-825 -165) + (3-65 X 5-11 2 - 7-69) = 90-22 incli units for each channel .-. Total J cc for compound section Allowing 90-22 x 2 = 18044 180-44 ~~ 6 /= 60 000 Ib. / sq. in. and F. of S. 30-07 inch units. 2-5. a .' a/r frr 60000x30-07 _. AAr . . Safe M = /Z CCi = - ^-= - = 722 000 Ib.-ms. Working similarly it will be found that M = 576 000 and 388 000 Ib.-ins. for the 5 inch by 2-|- inch and the 4 inch by 2 inch twin channels respectively. The Moment of Resistance diagram is given in Kg. 73 (b), from which it will be seen that if the pole is set 5 feet 6 inches in the ground it will be suitable for a lateral load applied 2 feet from the top of 722 000 24-5 x 12 2 460 Ib. It is therefore approximately equivalent in strength to a 32 feet by 9 inch wood " A " pole. IRON AND FERRO-CONCRETE POLES 14/ Ratio of Lateral to Longitudinal Strength. r . . . . 7 , 7 722000 010 . . (a) Available =-,. _ _ ^ = 2-12 6 ins. X 3 ins. -j v ' 170 160 X 2 [ (6) Utilised 2-12 576 000 5 ins. X 24 ins. ^ ^OOO^ 11 (a) 114000X2 388 000 4 ins. X 2 ms. , 260000 60 770 X 2 The design does not therefore make the most economical use of the sections. The distance between backs of channels may be in- creased to 20 inches nearly without exceeding the ratio of lateral to longitudinal strength of 4 to 1 allowed by the regulations, and the working load may be increased accordingly. The above calculations assume the two members to be parallel to one another, but in practice it is usual to incline them towards each other, mainly for aesthetic reasons. The moment of resistance diagram for the case in which the members are inclined, with .12 inches between backs of channels at the ground line and 8 inches at the 'top is shown dotted in Fig. 73 (6). With this arrangement it will be seen that the working load must be reduced to - 294 = 2 170 Ib. We will, however, continue our consideration of the parallel arrangement. * Bracing Required Above Ground. A suggested arrange- ment of the bracing is as follows : on each side of pole five diagonals in the 4 inch X, 2 inch bay, four in the 5 inch X 2|- inch and two in the 6 inch X 3 inch. The free lengths of the diagonals will then be about 25-5 inches in the 4 inch X 2 inch bay, 29-5 inches in the 5 inch X 2|- inch and 29-5 inches in the 6 inch X 3 inch. Pig. 73 (a) shows the diagonals on one side only. The diagonals must be designed to take the shearing load, viz. 2 4-60 Ib. Distance between centres of rivets is 9 inches approximately. Therefore the LOAD ON DIAGONALS 2460 2 cos oc 2x9 148 OVERHEAD POWER LINES We will use 1-| inch X 1|- inch X | inch equal angle, for which the least radius of gyration k = -29, . . 2=29* " k -29 iU1 '* Using Eider's formula (see p. 74) and a E. of S. of 2-5, the SAFE COMPEESSIVE LOAD 12 . 10 7 . A 12 X 10 7 X -687 (101-7) - 800 Size of Rivet. In addition to shear stress there will be a tensile stress in the rivet due to pulling up on contraction, and also a certain amount of stress due to bending. The F. of S. should there- fore be based upon the elastic limit. If d = dia. of rivet required, we have " X 27 000 = 4- 030 X 2-5, 4 and d = -69 in. A i^.-inch rivet will therefore be necessary, and consequently the calculations given above for strength of channels should be repeated, as they were based on |-inch rivets. The reduction, in strength however, will only be about 4 % and it is proposed to neglect it here. Tensile Stress in Diagonals. The alternate diagonals will be in tension. Allowing for loss of area due to |-|-inch rivet, the nett sectional area = -526 (-688 X -1875) = -526 -129 = -397 sq. in. .-. TENSILE STRESS = ~^r=- 10 150 Ib. / sq. in. -097 The B.C. require a P. of S. of 2-5 on the Elastic limit i.e. a limiting stress of 36000 _ . ....... . - = 14 400 Ib. / sq. in. a'O The 1-^ inch X 1|- inch X fVinch diagonal is therefore amply strong enough both in tension and compression. Buckling Strength of Channels. It is advisable to arrange IRON AND FERRO-CONCKETE POLES 149 tlie diagonals alternately on the two sides of pole to reduce the free length of the channel considered as a strut. 4 Consider, for example, the lower end of the 4 inch X 2 inch channel. The free length = 24 inches approximately and the distance between the centres of gravity of the two channel sections = 9-8 in. The B.M. at the working load = 260 000 Ib.-ins. (see Fig. 73 (6)), therefore the vertical compressive loading , = 260000 = MB001bi y*o To this should be added one half the dead weight load of the pole, pole fittings and conductors, which would be about 2 000 Ib. in all, therefore the total compressive loading = 26 500 + 1 000 = 27 500 Ib. For 4 inch X 2 inch channel, A = 2-085 sq. ins. and k (lesser value) = -703. ! .-. BUCKLING LOAD, P = (46 000 166,) M fc-085- 1 \ * ' = 840001b. and the JF. of S. = = 3-06. Forces Below Ground. It is the usual practice to set poles of this type in concrete. For calculation purposes the concrete is supposed to be a homogeneous body with a definite elastic modulus, in which case the stress distribution will be as represented by the area OABOD in Fig. 74 (b], page 150, the pole being supposed to pivot about the point 0, its centre point below ground. If / is the maximum compressive stress in the concrete, then the areas of the triangles OAB and OOD will be each equal to / X 6.x 33 lb T]ie Q^ of tliege triang i es w iu be f X 33 inches Zt from 0, therefore the MOMENT OF RESISTANCE OF CONCRETE = /.X 6X33X44^ 2 This must be equal to the BENDING MOMENT DUB TO THE LOAD which = 2 460 X (294 + 33), . 2 460 X 327 X 2 . OK ,, . *= 6X33X44 =18B)b./^.m. 150 OVERHEAD POWER LINES The concrete will stand 1 700 Ib. / sq. in. and there is, therefore, a high factor of safety in this -respect. Bracing Required Below Ground. The direct horizontal loading at the extreme points A and C (Fig. 74 (&)) will be 185 X 6 = 1 110 Ib. per inch run due to the reaction of the concrete. Since the horizontal forces must themselves balance as well as their moments 1110 LBS FIG. 74. Foundation portion of compound channel iron pole. (See Pig. 73.) we must also consider the pole top loading which may be taken as ., , , , 2 460 nform load of = 37 Ib. per inch run. Therefore the total t 4=^1110+37=] 147 Ib. and at 0=1110-37 = per inch run. The loading decreases uniformly down to IRON AND FERRO-CONCRETE POLKS If the total load on O.A. = W + ^ x z P .x then the load on. a length O.E. = W-j$ + .-. SHEARING FOECB AT E (Fig. 74 (b)). = W i i in v OQ Now W = 1U 2 X 66 = 18 300 Ib., .-. Substituting known values in above equation the shearing force at any point x inches above O - is 300 (i - ~ V 33V "2X S3 At the point 0, the maximum values occur, viz. 18 300 -|- .1. 230 = 19 530 Ib. from right to left and 18 300 1 230 =: 17 070 1I>. from left to right below 0. TIi.e shearing force diagram is shown shaded in Pig. 74- (6). It will be obvious that we need only to consider the lower value, Now if the pole is set in concrete and the interior Hj>aoo are well filled and rammed, the braces will be relieved of thcso very largo shearing forces, but we will assume that the interior is empty. A possible method of bracing to meet this latter condition in shown in Fig. 74 and consists of two 6 inch X -J inch plates at ground Him, two 6 inch X ;} inch plates at butt, two 12 inch X -Jjr inch plates at centre ; and four 3 inch X 3 inch X f inch diagonals (two only shown in Fig. 74). The free length of diagonals = *J&~~\- 22 2 = 24 inches approx. For 3 X 3 X | inch equal angle, A = 2-11 and k ~ -58 ! = -?* 41-3 k -58 ' .-. BUCKLING LOAD = (46 000 ICO X 41-3)2-ll = 82 600 Ib. The working load on each diagonal e Q 01 &< y 826QQ OVERHEAD POWER LINES Rivets. Assume f-roch rivets are used, and let n be the numbi j% required, then - X n X 27 000 = 22 800 X 2-5, and n = 4-8. Therefore five f-inch rivets would be required. Ib is, howeve more convenient to use four rivets, and these must be -*-f inc in diameter. Stress in Diagonal in Tension. Allowing for loss of are due to one -if-inch rivet, the effective cross-sectional area ( diagonal = 2- 11 - (-375 X -8125) = 1- 806 sq. ins. 99 00 .-. TENSILE STRESS = 12 600 Ib. / sq. in. 1* oOo This is well "below 14 400 Ib. / sq. in., the value permitted by th E.G. Regulations. The Factors of Safety so determined are on the pessimistic side since, although we have neglected the somewhat eccentric loading c the diagonals, we have also neglected the help given by the |-incl plates. Many other arrangements of the bracing will suggest themselves but it will be clear that if the pole is set in the ground in such i way that stability is to be ensured by horizontal reactions, then th shearing forces in the foundations require special consideration. The plates at the ground hne will not help much except to stiffei up the structure, especially during handling and erection, but th< two 12-inch plates at the centre will take an appreciable share of th< shearing load. Ground Reactions. Assume a concrete block 24 inches X 45 inches X 72 inches, as shown in Fig. 75. First consider the ground reaction under the block. The maxi mum rupture intensity at a depth of 72 inches = = 146 Ib. / sq. in. (Fig. 91, p. 181). The approximate weight of pole and conductors will be 2 000 Ib. and of the concrete block 6 000 Ib. therefore the direct compressive stress jj The load diagram is then as shown in Fig. 75 and the MOMENT OB ]! RESISTANCE due to this load I 146 7-93 !: = o X 24 x 42 x 4 = 278 000 Ib.-ins. = nW Ck I8150 o 7-93lbi against overturning & 2 093 000 ^ 43 " ^jtr 5,5Ss^ 154 OVERHEAD POWER LINES the block (shown dotted in Fig. 75) to take advantage of the superin cumbent earth pressure thereon. Ferro-Concrete Poles. Ferro-concrete poles have not beei used to any extent in this country, but they have been used a goo< deal on the Continent and in America. Their great weight is i disadvantage, some designs being 3 to 4 times as heavy as equiva lent wood or iron poles, although the ratio has been brought down t< 2 in some cases, notably in the " Harriot " design of pole, which i manufactured in this country. They require more care in handling than wood or steel poles and are frequently stressed more during transport and erection than they are likely to be subsequently ii service. They have to be constructed on or near the site, and this combined with their great weight, makes their use impracticabl< in difficult country. At the present time they cannot compete with wood poles iron a first cost point of view, but since they are practically everlasting and their maintenance charges negligible, and as the supply o: timber is unlikely to keep pace with the demand in the near future they may have to be seriously considered for distribution purposes in competition with iron. The design and construction of ferro-concrete poles should nol be lightly undertaken, unless the engineer has had experience oJ ferro-concrete work and has time to consider the matter carefully, The concrete mixture should consist of 1 Portland cement, 2 sand, 4 broken stones or gravel (pass f-inch mesh but retained by Y\-inch mesh). When well made, the crushing strength of such a mixture is about 1 000 Ib. / sq. in. in 7 days, 2 500 Ib. in one month; 3 000 Ib. in three months and 3 500 Ib. in six months. The weigh! is about 140 Ib. per cubic foot. Round reinforcing bars are invariably used, and, within limits, a large number of small bars are better than a few larger ones owing to the larger surface they provide for adhesion between steel and concrete. Angle and tee sections must not be used, as although they provide a large surface area compared with their cross-section it is practically impossible to ensure that the concrete is packed securely in the corners. The reinforcement should be covered with concrete to a depth which need not exceed 1 inch, but should be at least equal to the diameter of the bar. The bars should be free from loose rust. To illustrate the principles involved, a simple example of a IRON AND FERRO-CONCRETE POLES 155 3?:o. 70. Ferroconcrete pole. 156 OVERHEAD POWER LINES successful design, shown in Fig. 76, will now be briefly considered. The following assumptions are made : (1) The pole is subjected to pure bending and the total compres- sive load on one side of the neutral axis is equal to the total tensile load on the other. (2) Plane sections remain plane after bending. (3) For the concrete, as well as for the steel, the stress is pro- portional to the strain. (4) There is no slip between the concrete and the steel. This is true except in the case of very short columns ; also the coefficients of expansion of steel and concrete are about the same, and therefore no appreciable stresses are set up by ordinary changes in tempera- ture. (5) The whole tensile load is taken by the steel reinforcement. (6) The area of the reinforcement is so small that we may assume the stress constant over it. The following constants will be taken : Ultimate tensile stress Steel 65 000 Ib. / sq. in. ,, Concrete 250 ,, Ultimate compressive stress Steel 55 000 ,, Concrete 2 500 Modulus of elasticity Steel 30 . 10 6 Concrete 2 . 10 G Safe maximum adhesion between concrete and steel 100 In structural work it is usual to design for a Factor of Safety of 2 for steel based on the elastic limit, and a F. of S. of 4 for concrete based on the ultimate compressive stress. We will assume, however, that the bending theory holds up to the ultimate stress, and allow a F. of S. of 3-5 on the structure as a whole, as required by the E.C. Regulations. The following notation will be used : Tensile stress Steel f st Concrete f ct Tensile load ' T s Compressive stress / Concrete f cc Compressive load C s C c Modulus of elasticity E s E c Area of cross section A* ,, A f IRON AND FERRO-CONCRETE FOLES 157 Fig. 77 (a) shows a cross-section at the ground line of the pole under consideration. For simplicity we will neglect the web portion and the eight ^-inch rods. To calculate the strength of the pole we must first find the neutral axis, NN'. Let this be x inches from the edge of section on com- pressive side. Then, as there is to be no slip, we have (Fig. 77 (&)) Maximum compressive strain in concrete A A 1 AS x Compressive strain in steel CC 1 CS x 1-3' i.e. Now E c Isc E, _ 30 . 10" E ~ 2 . 10 e 15, 1-3 x /fi/i>/sjr, (a.) Section. (c) Stress and limiting / to 2 500 Ib. / sq. in., / = 15 X 2 500 (l - ) = 37 500 - Ib. / sq. in. \ * f UU Area of Steel = 2 X ~[Y|) + (^} j = 1-6 sq. ins. .-. TOTAL COMPRESSIVE LOAD ON REINFORCEMENT = (L = 1-6 ^37 500 - 4 158 OVERHEAD POWER LINES Similarly tensile stress in steel on tension side =37 600=37 50oi- 7 _ l X ,-. TOTAL TENSILE LOAD ON REINFORCEMENT _ 37 500 = IMo _ 60 We must now find the compressive load on the concrete. The stress at the outer edge of iange = 2 500 Ib. / sq. in. and at the , % _ g.gs inner edge = (2 500 X - ) Ib. / sq. in. & \ x J ' i The average stress is the mean of these two values, viz. 2500 I - _ - = 500 - Ib. s . in. Area of concrete flange = 3-5 X 12 = 4-2 sq. ins. (neglecting small loss of area due to reinforcement). .. TOTAL COMPRESSIVE LOAD ON CONCRETE Ib. 422 500 - * = 105 000 X J \ X J Now by assumption C s + G = T s , i.e. 60 000 - -_ + 105 000 _ = _ 6 o 000. a; xx Whence x = 6-15 inches. 4.0 7KA / = 37 500 - =^p = 29 570 Ib. / sq. in. O, = 29 570 x 1-6 = 47 400 Ib. f at = !^? _ 37 500 = 76 500 Ib. / sq. in. CThis stress is too great, but we will continue the calculations and ^w for this later.) IRON AND FERR.O-CONCRETE POLES 159 TI = 76 500 X 1-6 = 122 500 Ib. f e at outer edge of concrete flange = 2 500 Ib. / sq. in. $ _ 3.5 / c at inner edge of concrete flange = 2 500 X - Q5 K.TK _ 9.K = 2 500 X * = 1 080 Ib. / sq. in. 6-15 ' A O c = 42 x 1 080 + 42 x 2 5 7 l 8 = 75 100 Ib. 2i (See Fig. 77 (c).) Total compressive load = C a + G G = 4-7 400 -|- 75 100 = 1.22 500 Ib. Moment of Resistance. Let the centre of pressure of the compressive loading be y inches from the outer edge of concrete flange, then taking moments about this edge, we have, dividing the stress diagram for the concrete into i octangular and triangular portions (Fig. 77 (c)), Q.K 1 A r>(\ <>. 122 500y = 4-7 400 X 1-3 ~|- 1 080 X 42 X -.- + ~ - X ^ X ' - 22 o = 61 600 + 79 400 -j- 34 800. W1 175800 , , . , Whence y = j^-^ = 1-43 inches. .-. Centre of compressive loading = 0-15 ~- 1-43 = 4-72 inohoa from neutral axis. Now centre of tensile loading = 20 1-3 6-15 12-55 inches. from neutral axis. .-. Arm of resisting couple = 4-72 -f 12-55 = 17-27 inch OR. and MOMENT OF RESISTANCE = 322500 x 17-27 = 2 120 000 Ib.-ijw. But this based on f at = 76 500 Ib. / sq. in., whereas the ultimate strength is assumed to be 65 000 Ib. / sq. in. only. The moment of resistance, therefore, must not exceed 2 120 000 x - 1 800 000 Ib.-ins., and allowing a F. of S. of 3-5 the BENDING MOMENT must not exceed 1 J 160 OVERHEAD POWER LINES The gross maximum working load at a point 2 feet from top of pole (i.e. 30-5 feet from ground level) 30-5 X 12 Alloy/ing for wind pressure on pole, the nett safe working load due to wind on wires = 1 410 140 = 1 270 Ib. The pole is approximately equivalent to a 40 foot/ 8 inch " A " pole. In the above treatment we have neglected the direct com- pressive loading due to the dead weight of the pole and conductors, which would together be about 5 000 Ib. The cross-sectional area of the concrete = 116-5 sq. ins. and the compressive stress, if as- sumed to be taken wholly by the concrete, would be increased by 5000 , K11 . . =:451b./sq.m. On the other hand, owing to the limit imposed by the strength of the steel on the tension side, the maximum working compressive stress in the concrete due to bending is - X =^~K = 607 Ib. / sq. O"0 (O'O in. only, instead of --- 715 Ib. / sq. in., which is the safe maximum ; O'O moreover, the eight J-inch rods have been neglected, and at the end of six months the strength of the concrete will have increased by 40 %. Stress in Concrete on Tension Side. We have neglected the tensile strength in the concrete in our calculations, but if, as we have assumed, there is no slip between the steel and the concrete, there must be a tensile strain in the concrete equal to the tensile strain in the steel. ,, , 65 000 . . If f,t = -Q- ft- / sq. in. 4.1. x 65000 2.10 6 - , nl , . then / ot = -g-g- x ^Y QQ = 1 240 Ib. / sq. in. This is an extreme figure, since in the initial stages the concrete takes a part of the tensile load. Now the ultimate tensile stress of concrete is only about 250 Ibs. / sq. in. Hence it is evident that in service the concrete cracks on the tension side, but owing to the adhesion between the "Fia. 78. Eerro-concrete pole. Overlapping joints in the reinforcement are shown to introduce the calculations involved, but welded joints would invariably be used in modern practice. 11 162 OVERHEAD POWER LINES steel and the concrete, the failure consists of a very large number of small hair cracks extending along the whole length of the pole. Moisture from the air enters these cracks and automatically seals them by acting on minute unhydrated particles of cement. Thus the steel rods are protected from corrosion. STRENGTH OF JOINT (Fig. 78). Length of Joint = 39 ins. Centre of Joint = (39 19-5 2)12 = 210 inches from point of loading MAXIMUM BENDING MOMENT AT CENTRE OF JOINT 210 = 515 000 X STT = 296 000 Ib.-ins. obu Arm of couple =11 inches approx. .-. Tensile load on joint = = 26 900 Ib. Surface area of reinforcing' rods = 39 X TT (f -f- f -(- -/ F -(- T V) = 292 sq. ins. .. Max. longitudinal stress between steel and concrete 26900 . = "292" = ^ Sq ~' m * The safe maximum = 100 Ib. / sq. in. 163 CHAPTER IX. ANGLES AND TEEMINALS. Angles. We will calculate the stress in the stay wire, etc., making the following assumptions : (!) Three -05 sq. in. copper conductors and an earth wire ar- ranged as in Eig. 26, p. 45. (2) 34: foot / ll- inch pole buried 6 feet. (3) Span length, 250 feet. (4) Angular deviation, 10 degrees. Under these conditions the lateral load on each insulator = P -f 2 . T sin - (p. 60) A = 177-5 + 1 457 X -174 = 4-31 Ib. The total load due to four wires = 431 X 4 = 1 724 Ib. The centre of pressure is approximately 336 28-75 =- 307-25 inches from ground level. The bending moment on the pole at the ground level due to wind pressure on the pole itself = 31 600 Ik-ins., which is equivalent to e\ -t C\f\r\ a load of ~~j-~ = 103 Ib. at a height of 307-25 inches. {The wind load on the pole fittings is small and may be neglected.) The total LATERAL LOAD is therefore P=1724-|- 103= 1827 Ib. Stay (Fig. 79, p. 164). In order to keep the stay wire well clear from the line conductors it will be advisable to fix it a little below the centre of pressure, say 36 inches from top of pole. It will not usually be necessary to bother about any small dif- ference between the centre of pressure and the point of attachment of the stay, but we will allow for it in this case to illustrate the prin- ciples involved. 164 OVERHEAD POWER LINES ANGLES AND TERMINALS 165 If PJ is the horizontal component of the reaction clue to the stay wire it must have such a value as to prevent any deflection of the pole at the point of attachment. It must be emphasised here that as far as the load on the stay is concerned no allowance can be made for the strength of the pole itself, which acts simply as a strut. The pole deflects when stressed due to lateral loading and if this occurs it means that the stay wire or its anchorage has failed. It can be shown that to prevent any deflection of the pole, P x .0 rr . must be equal to ( -| }P, in which H is the height of pole to centre of pressure and S the height of pole to point of attachment of stay. In our example 3X 307-25 2X300 = 1-04 X 1 827 = 1 900 Ib. Now whether we use a stay or a strut, the further from the pole we are able to fix it the less the loads to be dealt with. We will assume for purposes of calculation that 15 feet spacing is available. Then the A/25 2 4- 15 2 TENSION IN STAY = v ,~T - x 1 900 15 9Q-9 = Lf x 1 900 = 3 700 Ib. 15 Referring to Table XVI., p. 170, we find that a 7/-16 stay wire will meet the requirements. 25 COMPRESSIVE LOAD ON POLE = x 1 900 = 3 170 Ib., 15 to which must be added the dead weight of the conductors, pole fittings and of the pole itself, which would be about 1 850 Ib., making a total of 3 170 + 1 850 = 5 020 Ib. The buckling strength can be checked as explained later for terminal poles (see p. 172). Intensity of Ground Pressure under Butt. Taking the butt diameter as 12 inches this 5 020 X 4 , , K , = = 44-5 n>. / si- m ., 166 OVERHEAD POWER LINES 431 LB3 CENTRE OF PRESSURE P which is just below the safe maximum (see Fig. 91, p. 181) at a depth of 6 feet. But as it is most important to avoid settlement it will be advisable to distribute this load over an area of at least 2 sq. ft. bv means of a block of concrete or creosoted timber. 25 The stay wire makes an angle of tan" 1 = 59 with ground level, 3 700 so if the stay block is buried 4 feet, an area of ^-^ = 2-31 sq. ft. is nec essary (see Kg. 91, p. 181). A stay block 9 inches X 4 inches X 3 feet 6 inches long is sug- gested. Strut (Fig. 80, p. 164) shows the usual form of construction. Struts are seldom used. They are rather unsightly and more expensive than stays and in the survey of the route every endeavour should be made to render struts unnecessary. They may sometimes be advisable, however, if stays are likely to be attacked by im- purities in the atmosphere, especially near chemical works. In this connection it may be noted that a galvanised high purity iron stay wire has recently been standardised which _. . is more durable than sal- lie. 81. . . 5 vanised steel wire, (bee B.S. Specification 1831927.) Taking the same conditions as assumed above for the stay, we will suppose the strut to be fixed 12 inches below arm bolt. Bending Moment on Pole at Weakest Section. This will be where the strut is attached (Fig. 81) = 862 x 20 + 431 X 62 = 44 000 Ib.-ins. Diameter of pole at this point would be about 8-5 inches. JujL- j&.$/$M*>'> f * v >'-'ffr/i.w'<y.WJk ANGLES AND TERMINALS .-. Moment of resistance 800 X rr X 8-6* = _ - 32 ~ 32 There is therefore ample strength here. The horizontal reactioi due to strut must - 41827 2X 283 = 1-13 X 1 827 = 2 060 lb., neglecting wind pressure on strut itself. Assuming strut to be fixed in ground 8 feet from pole, tho upwaix pull on pole = 2 060 X H = 6 070 lb. 96 Allowing for dead weight of pole, etc., the NETT UPWARD PIILI ON POLE FOUNDATION == 6 070 1 850 = 4- 220 lb. Cross blocks will, therefore, be necessary to HOLD Tim POL DOWN. If they are fixed 4 feet below ground level their arc A OOQ must not be less than f- = 3-38 sq. ft; (see Fig. 91, p. 181). A suggested arrangement is shown in Fig. 80 employing tw 8 inch X 4 inch X 3 feet cross blocks and two 2 inch X 12 inch > 2 feet kicking blocks. The upward pull on strutted poles is a good deal greater than i usually supposed. Compressive Load on Strut. TH3 = 2060x^3=2060x1? =0420 11, Assuming the strut to have one end fixed and tho other pivote and neglecting the restraint due to the buried portion and to tl tie, bolt, the mean diameter required will be given by tho equation r> 2-257r^/ _ 2-25 X 7r z X 1-2 X 10 X TT X D* _ r , 9ft B== --- ____ ^ 64 X 29-5* X 12 2 ' ~" '"" X ' __ 6 420 X 3-5 X 64 X 29-5 2 X 12" Wlience ~ 168 OVERHEAD POWER LINES Length, of strut required is about 30 feet, allowing a buried depth of 4 feet 6 inches. A 30 foot / 8| inch pole is suitable. Ground Pressure under Butt of Strut. The strut itself weighs about 550 lb., and assuming for simplicity that the whole of this acts in the direction of the strut the total load on the ground tinder the strut = 6 420 + 550 = 6 970 lb. The symmetrical ground pressure at a depth of 4-| feet must not exceed 4~ = 32-9 lb. / sq. in. (Fig. 91 p. 181). TT X 9 - 35 2 Area of pole butt = - - - = 69 sq. ins. approx., therefore the ground under pole will only support 32-9 X 69 = 2 260 lb. Consequently cross blocks must be provided to take a load of 6 970 2 260 = 4 710 lb. A creosoted wood or concrete block about 18 inches square, under the butt, would meet the case, but it is pre- ferable to fix a cross block, as the strut is then well anchored and will act as a stay as well, should circumstances ever require it to do so. If the cross blocks are fixed with their under surface an average 4710 of 3 feet 6 inches below ground level, an area of _ - = 1-65 sq. ft. 2 860 will be necessary. Two 8 inch X 4 inch X 3 foot blocks are suggested. Strength of Scarf -Joint. The length of scarf on the strut will be about 2 feet and it is therefore advisable to use two bolts, one at bottom end of scarf and the other about 12 inches above. The vertical load taken by the scarf joint = 6 070 lb. If the strength of the joint so constructed is checked as shown on page 122 it will be found to be distinctly weak, depending as it does simply on two bolts, and the help given by friction. The moment of resistance of the pole itself can only come into play if the strut foundations give, and this is forbidden by the regulations. The fact that this type of joint seldom fails can only b ascribed to the rare occurrence of the hypothetical loading conditions. To make a really satisfactory job and provide a factor of safety of 3-5 an oak block should be fitted as in the case of an " A " pole, or the strut should be let into the pole for an inch or so at the top. ANGLES AND TERMINALS 16! The pole invariably has sufficient margin of strength, to permit o this. Use of Rutter Poles at Angles. Kef. to Fig. 65, page 132. shows than an 8-inch Rutter pole has a safe working load of about 2 400 Ib. at a point 26 feet from ground level and would therefore be quite suitable for the conditions considered above, thus obviating the use of stay or strut. Rutter poles are now being largely employed for angles, and they are particularly useful in cases where space is limited. In all cases angle (and terminal) poles should be given a slight " rake," i.e. an inclination away from the pull to allow for the small " give " in the foundations which occurs in the initial stages when the load is applied. Terminals. We can seldom use cap fittings on terminal poles, and it is advisable to allow greater clearances between conductors, therefore somewhat higher poles will be required if the triangular arrangement is maintained. To obviate the use of longer poles the three conductors can be placed in the same horizontal plane, but this does not make such a neat job. We will base our calculations on a 36 foot / ll'l inch pole buried 6 feet, the stay wire being fixed 26 feet from ground level and the conductors arranged as shown in Fig. 26, page 45. The maximum longitudinal pull on the pole under basic loading conditions = 1 457 X 4 = 5 828 Ib. If the stay is fixed in the ground 15 feet from foot of pole, the TENSION IN STAY WIRES (Fig. 82) T 5 828 A/26 2 + 15 2 15 = 11 660 Ib., and the COMPRESSIVE LOAD ON POLE 26 5 828 X 15 lOHOlb., to which must be added the weight of pole, pole ironwork and of half a span of ice-loaded conductors and earth wire. This 'will approach 1 500 Ib. 170 OVERHEAD POWER LINES Therefore TOTAL COMPEESSIVE LOAD ON POLE = 10 110 H- 1 500 = 11 610 Ib. approx. To provide the required factor of safety of 2-5, the breaking load of stay wires must not be less than 11 660 X 2-5 = 29 150 Ib. Table XVI. gives particulars of the most common sizes of stay wire. TABLE XVI. Particulars of Stay Wires and Stay Rods. Breaking Safe Work- Weight Load, ing Load, per Foot, Ib. Ib. Ib. Galvanised Steel Stay Wire 7 / -08 2450 980 120 ^ 4/-16 5 COO 2240 274 1 B.S. Spec. 7/-1C 9800 3920 479 f 183, 1927. 19/-16 26000 10 640 1-310 J StayR ods wi liTigl tenets in. H 10G50 15 900 4260 6 360 ] British 8 21 000 29100 8 400 11640 > P.O. J Standard. The figures in the table assume a breaking stress of 70 000 Ib. / sq. in. for the galvanised steel stay wire and about 52 000 Ib. / sq. in. for the galvanised wrought iron stay rods (at bottom of threads). It is not advisable to use wire of greater tensile strength, as it deteriorates more rapidly. Instructions for Attaching y-Strand 0-16 Dia. Stay Wire to Thimble. Bend the stay wire to form, two knees 7 inches apart, the first of these knees being 23 inches from the end of the wire. Bend the wire between the two knees round the thimble, using ' ' O the stay tool to draw it close into the groove. Unstrand the free end, straighten out the wires, pick out one end for the first lap, and loosen the tool whilst the wire is passed underneath it, again grasp the remaining wires with the tool and place them symmetrically parallel with and around the main strand, so that they will bind into it without spoiling its circular shape. Grip with the tool and revolve the latter with the free wire under the hook on the thimble side of' the tool. This wire should make eight laps. Treat the other wires the same way, as shown in Fig. 83. The projecting short ends of each wire (which should not be more than \ inch long) must be worked in by grasping the splice with the ANGLES AND TERMINALS 17 tool (the ends being within the hollow of the tool) and turning th tool over each end until it is worked in. Method of Attaching 7-Strand 0-16 Stay Wire to Pole.- Lap twice round the pole, secure by half a dozen No. 4 S.W.G staples and finish off the loose end on to the standing part in th Stavj Tool. Qir - 8 La,|o3 around main., binding in 6 loos* - 7 - 7 Total 49 Lajos .. 3 ITiG. 83. Making off 7 -strand '16 inch dia. stay wire on thimble. manner described above. Alternative methods are shown in Fi 79, p. 164. For a breaking load of 29 150 Ib. it will be seen that three 7/v stay wires are required, or their equivalent. (For symmetry, using an H. Pole, four 7/-16 stay wires won probably be used in this case.) 172 OVERHEAD POWER LINES If, however,, a distance of 26 feet from the foot of the pole is available, the load on the stay will be reduced to . 26 In this case three 7/-16 stay wires will give a F. of S. of . . - -.-. 3-56, and two such wires a F. of S. of 2-37 only, which is hardly sufficient, but in practice they might possibly be made to satisfy the requirements by fixing them a little further from the pole. The direct compressive loading on the pole will in this case be reduced to 5 828 + 1 500 = 7 328 Ib. The effects of this compressive loading will now be considered. Resistance of Earth under Pole to Direct Compres- sion. The intensity of pressure on the earth under the pole when the stay is 15 feet from pole 11610 . 103 Ib. / sq. in. approx. From Fig. 91, page 181, it will be seen that the maximum safe intensity of pressure at a depth of 6 feet is about 58 Ib. / sq. in. It will therefore be necessary to distribute the pressure by means of a block of creosoted wood or concrete. Strength of Pole to Buckling. It was pointed out on page 119, when considering the buckling strength of the compression member in. " A " poles, that no great precision could be claimed for the calculations. The same remark applies here. It is difficult to procure poles perfectly straight and to erect them. exactly vertical. The loading is not concentric, and the conductors, sarth wire and stays are not all attached to the pole at the same ^oint. Moreover, the " end conditions " and the " effective " aigth of the pole considered as a strut can only be guessed at. Lateral deflection of the pole in strong winds further complicates lie problem, but this can be obviated by splaying out two of the erminal stays as indicated in Fig. 84. It will be realised, therefore, that exact calculations are impos- ible, taking all the relevant factors into consideration, but Euler's ANGLES AND TERMINALS 173 formula for a strut hinged at both ends, taking values for E, J and L as defined below will be found to give results agreeing closely with, experiment. If E = Modulus of elasticity = 1-2 X 10 6 Ib. / sq. in., J = Moment of inertia (in inch units) of the cross-section of the pole about half way up, L Overall length of pole up to point of loading (in inches) (the reaction of the ground to the buried portion being neglected), B BUCKLING LOAD IN POUNDS, 7T 2 . E . J then -D = ^5- . FIG. 84. In our example Mean diameter of pole = ^ = 10 inches approx. 77 . 10 4 = 4-91 inch units. __ 64 64 L = (36 - 4)12 = 384 inches. - L * 3342 (a) With stays fixed 15 feet from pole 39400 (b) With stays fixed 26 feet from pole The pole selected would, therefore, be quite suitable for case (b), but is not quite strong enough for case (a}. It is to be remarked, however, that the minimum specified dia- meter of the standard pole has been assumed. A consignment will 174 OVERHEAD POWER LINES POLES Trussing TacMe Stng/e, comp/e/&. consfsfo of i- TrtrSS fl//j$ *- / B/ocfa for " " = 8 Truss ffgi/s A //TC/I . Tte Boffe - G. /. Tub/ng /' ieng/hs - Wasfie D/a/?>efer of Po/es <* c/s's/Jn oehveen cen/fes fnuaf he iSpeetfject tvJien ofe/er/ng 'ftng <?/ ' flo/e Pio. 85. Typical terminal "H" pole. (Por details of pole fittings, see Fig. 8G.) ANGLES AND TERMINALS il 176 OVERHEAD POWER LINES invariably include some poles with, diameters appreciably above the minimum and these would naturally be earmarked for terminals and angles. In practice, wayleave considerations tend to shorten up the stays and it may be found necessary to use non-standard single poles of large diameter or, preferably, " H " poles. The latter are more desirable from electrical considerations as the greater space available makes it more convenient for dealing with conductors and stay wires and for fixing cable terminating boxes, switchgear, etc. Figs. 85 and 86, pages 174 and 175, show a* suitable design for the line under consideration. Stay Anchorages. It is often assumed that the resistance offered to a stay block is equal to the weight of earth contained in the frustrum of a pyramid of which the smaller base is the -stay block itself (assumed horizontal) the side faces make an angle <f> with the vertical and the larger base is the ground surface. . The weight of such a frustrum of earth (Fig. 87) is given by the following expression : W = -j- 2d tan 0) + Ib 2d tan 2d tan in which d = depth buried, I length, and 6 = breadth of block, < = angle of repose of soil and w = weight of soil per unit volume. This rule does not appear to have a theoretical basis of any sort and the fact that it gives reliable results in practice (if b is not less than (say) 8 inches) is undoubtedly due to the neglect of two other factors, viz. the COHESION of the soil and the INCLINATION or THE PULL. Consider a vertical retaining wall AB (Fig. 88) with horizontal ground surface AC. It can be shown that if the wall is moved horizontally towards the soil it retains, rupture takes place along the line BC, making an angle of (45 + |j ) with the vertical. \ 2/ ANGLES AND TERMINALS 177 When, a stay block is placed at B in the usual way so as to bear on undercut soil., the cohesion of the soil to the left of the vertical AB is destroyed at least temporarily and may therefore be neglected. If it be conceded that the line of rupture when the anchorage fails may possibly be the line BC as denned above, then the vertical rupture intensity of the soil ab the depth d will be due to the reaction of the triangular mass of soil ABO. This reaction will be propor- tional to the weight of the mass, together with the cohesive force tending to prevent separation along the line of rupture. Anything like a complete expression for this reaction would be too cumber- some for practical use and, moreover, owing to the want of homo- geneity of the soil and the uncertainty of the values of the specific weight, angle of repose, etc., it is fatuous to attempt close accuracy. P FIG. 88. Suppose the stay block to be one foot wide, that the surface is horizontal and the pull vertical. The weight of the volume of soil of triangular section ABO and one foot in thickness, which con- tributes to hold down one foot lenth of the block Ib. in which w = weight of soil per cubic foot in pounds. d = depth in feet. (j> = angle of repose. Now the cohesive force can be shown to be a function of the height which the soil will stand when freshly cut, and also of the angle of repose, but to simplify matters we will assume that the effect of cohesion is apparently to increase the specific weight of the soil. With the above assumptions, therefore, the vertical intensity of 178 OVERHEAD POWER LINES pressure on tlie stay block when the anchorage is on the point oJ giving way may "be written p = Kwd*h tan (45 + ) Ib. / sq. ft. FIG. 89. h being the height in feet which the soil will stand when freshly cut and K some constant yet tc be determined. Now consider the state o: affairs when, as is usual, th( pull is not vertical. Let ab (Fig. 89) represenl section of stay block of unii area, the inclination of the pul being cc with the grounc surface. Assume the ratio of th< horizontal rupture intensity of the soil to the vertical ruptun intensity to be (N + 1). Then if r equals the pull on the bloc! at which it gives, we have r . ab = p . ac . sin cc -f- (N -f 1) . p . be . cos oc = ^{sin 2 oc + (N 4. l) cos 2 oc} = jj{l -f N cos 2 oc} = p{l + N(l - sin 2 oc)}. Substituting the value ioxp found above we get 45 - sin 2 oc)} Ib. / sq. ft. It is realised that the above reasoning is open to criticism i] many respects, but it is interesting to compare it with the following very similar formula given in a paper in the Journal of the Institution of Civil Eiu/ineers, in 1912, by Capt. (now Lt. Col.) C. E. P. Sankev E.E. ret. : r = Kwd z h sin 2^(1 -f-..#(l - sin oc)} Ib. / sq. ft. It will be found that, providing suitable constants are choser the results^ obtained with one formula are not very different fror. those obtained with the other. In the paper referred to, the experimentally determined value for the constants K and N to suit the latter formula were given a 049 and 2-37 respectively, but admitting the difficulty of assignin ANGLES AND TERMINALS 179 precise values to w, h and <j>, the following simplified expression was suggested as being quite close enough for practical purposes. As it has proved reliable for many years, its use is recommended. r wd z h sin 20(1-5 sin oc) Ib. / sq. ft. Although based on consideration of a block 1 foot broad, experi- ments show that the expression can be used for any breadth up to several feet. It has also been verified that the effective breadth of a round log is equal to its diameter. The holding power is reduced if the soil becomes wet. This can be allowed for by reducing the value of /t. Approximate values of the constants for various soils are given in Table XVII. TABLE XVII. Constants for Various Soils. w, Ib. h, feet. *, degrees. Mud .... 90 Loose dry earth (loamy soil) 90 0-1 25 Ordinary surface earth (loamy s jil) 90 1-3 25 Well-drained earth (loamy soil) 100 5-10 30 Moist earth (loamy soil) 100 1-3 40 Very wet earth (loamy soil) 100 0-1 15 Ordinary dry clay . 120 9-12 30-35 Damp clay, well drained 120 4-8 45 Wot clay. 120 0-3 15-20 Clean dry sand 100 0-1 30-35 Wet sand. 100 1-5 25-30 Clean gravel 110 0-1 40-45 Damp shingle 100 . 40 Loam, with gravol 110 1-3 25 fcsand, with gravel 110 0-1 25 Clay, with gravel 110 1-3 30-40 For well drained, loamy soil we may take as conservative values w = 90, h = 5 and ^ = 30. Substituting these values in the for- mula and allowing a factor of safety of 2-5 as required by the E.G. Regulations, we get r = 156<Z a (l-5 sin oc) Ib. / sq. ft. Values of r for various depths and angles are plotted in Fig. 91. In our example we will for simplicity deal with one stay anchorage ' only, assuming that it takes one- third of the load. 180 OVERHEAD POWER LINES It is advisable to have in all cases at least two distinct stays and anchorages, separated by not less than 6 feet. Case (1) Stay 15 feet from pole, T = il|?? = 3 890 lb., oc = 60. <j 8240 Case (2) Stay 26 feet from pole, T = ^ = 2 750 lb., oc = 45. Assuming a depth of 5 feet, we find by reference to Fig. 91 that the safe working load per square foot equals 3 100 lb. at 45 and 2 500 lb. at 60. Therefore the area of stay block required n ,- . 3 890 , K _ Case(l) =-- =1-56 sq.ft. Case (2) = ^ = 0-89 sq. ft. Theoretically, if the stay block is 9 inches wide it need not be longer than 2-08 feet in case (1) and 1-19 feet in case (2). X" Now consider the strength of the block itself. Strength of Stay Block (Fig. 90). With a load of 3 890 lb. and a stay block 2-08 feet by 9 inches, what should be its thickness? First consider bending : 9 QQA Distributed load per inch length = - = 156 lb. a ,,72 1 KR 1 4-R^ Maximum B.M. (at centre) = - - = = 12 150 Ib.-ins. bd g Moment of resistance of section of block = M = fZ = f . J J Q Q ,72 = 7 800 . ~ Ib.-ins. 6 Allowing a factor of safety of 3-5 12 150 .3-5.6 _ ~~ 7 800 . 9 ' ~ whence d 1-91 inches minmium. ANGLES AND TERMINALS 181 Maximum shear stress 3890 113 Ib. /sq. in. 9 X 1-91. X 2 This is not excessive. An iron washer 6 inches X 6 inches X mcn should always be used under the head of the stay bolt, to distribute the pressure. This washer is neglected in the above calculations and the values are therefore on the safe side. A stay block 24 inches X 9 inches X 2 inches thick will obviously satisfy requirements, but as the cost of the stay blocks is a very small percentage of the overall cost of the line there is no need to cut the nooo 15 30 45_ 60 75 90 ' Inclination of Stay Wire with Ground (Deg.) Fro. 91. Strength of anchorages in good soil. dimensions too close and in practice blocks less than 3 feet long and 4 inches thick are seldom used. It must be remembered that the stay block performs a very important function and failure of an anchorage may result in the destruction of a large number of supporting poles. It is out of sight and usually out of mind, it may not be buried fully to the depth prescribed, it may be damaged a little when fixing, the creosoting may be imperfect, and the rate of decay more rapid than antici- pated, and, moreover, we can seldom fix precise values for h and <}4. In conclusion, the use of thin sheet-iron stay plates may be referred to briefly. 182 OVERHEAD POWER LINES Theoretically, tlie uniformly distributed breaking load o iron plate 24 inches square, if assumed to be supported at the ce only by a 6 inch X 6 inch washer under the bolt head, is a 5 000 Ib. for a plate & inch thick and 9 000 Ib. for a plate thick. Actually plates will support greater loads than these, since bending which occurs causes a redistribution of the load as ii creases, with a maximum value at the centre, and further, in initial stages, the reaction of the earth below the plates opj the tendency to bend. Experimentally, a plate 24 inches X 24 inches X fV inch found to stand up to 14 000 to 16 000 Ib. temporarily and gsw 9 000 to 11 000 Ib. when sustained. Figures for a 24 inch > inch. X i inch plate are 20 000 and 15 000 approximately. It may be inferred from this that the safe maximum wor load for a 24 inch X 24 inch plate is -5-^- = 4 000 Ib. when -^ i'D thick and ' r == 6 000 Ib. when 1 inch thick. 2-5 * To take full advantage of the bearing surface and use the va given in Fig. 91, it will clearly be necessary to reinforce the pi radially. 183 CHAPTER X. CONDUCTORS OTHER THAN COPPER. CONSIDERATION of other conductor materials has been purposely postponed, because comparison between the various more or less suitable materials is very much influenced by their mechanical properties as well as their electrical. The main desiderata in a conductor material are : 1 . Low price. 2. High specific electrical conductivity. 3. High tensile strength. 4. Chemical inertness to atmospheric effects. 5. Ease of erection and jointing. Particulars of various conductor materials are given in Table XVIII. Pure hard-drawn copper most closely satisfies all the required conditions. It is still the best known and most widely used material and at its present price it is likely to remain so both for low voltage and high voltage distribution. It lasts indefinitely in use under ordinary atmospheric conditions and it has a high scrap value. Copper Alloys. Owing, however, to the large ratio of sag to span length necessary with the smaller copper wires for high voltage lines, a conductor of greater tensile strength is desirable for the transmission of small amounts of power. Bronze. The addition of a little tin and silicon to copper pro- duces an alloy known as " bronze " which has a much greater tensile strength than copper, but unfortunately the gain in strength is only obtained at the expense of a reduction in conductivity. The standard P.O. silicon bronze (B.S. Spec. 1751923) has a tensile strength of about 100 000 Ib. / sq. in., which is a 70 % increase on that of H.D. copper, but its conductivity is reduced by more than 100 %. For equal conductivity it costs more than twice as much as pure copper. 184 OVERHEAD POWER LINES 8 O O CO rH G2 o 14 f rH CO CO Tt< ^ c^i >p cp o o . n C^I rH CO CO IT !> CT CO a "S 1 ^ & ** (j O *-* S B: S tc m 3 t ** O O O o o o IO X CO 1 O O rH CO O IO CO V <N O i 1 00 rH 4< ' o ' ob co o CO <N rH OJ g 'H -M 00 IO MP O o o 8 1C X O O OO rH CO rH O C<] 4< ' ob ' t-- w CO O O cp o C5 CS 1 resistance |^J o o o IO o o <N o CO 1 rH CO CO "* y !N IO CO ? t^ CO >O IO CO rH rH fl cd .s CT g ^10 S; M 8 o IO 8 1C <M o H X 14 t o 05 CO >* 1 ~ l CO 00 j , _, {gj CD cp C^l rH * ^ 6 6 rH O CO O C<( CO rH rH it 45 000 Ib. / s rili o o o CO 8 X o 1 1 1> v C5 M< rH Ol OO A O CO CD -H (N rH IO !> rH 1 & S rH OS ^ rH rH rH O rH u?g !*-. ll o o o 1 I >o o X O rH CO XCN O3 T^ C3^ Ci CO rH OO i I O O O CO CO ^1 O2 M d 00 rH O ^ iff o o o o IO CO rH O v M ' >o ^ <N t- CO 1> ^ft 1O CO O OS O O rH O CO (N ,0 cS CO co Iff t- 1 I ^ M ^ ^ rH rH U^ iipv o o o o o o o s t CO CO rH Ol H O3 IO IO \x CM *^ CM rH OO IO I-H 5? 00 -* o 00 CO O CO rH TH rllU |||V o o o o o o o X 2 os co oj co o ^ CM IO CO CO CO O Jjjj 00 s X SSI O W rH CO 1 a^ 10 -5 o o o o 1 ( X 1 O IO rH N O I- 1O v CO rH ^ rH -rfl IO l- >O O3 f oo oo . ?cbg "^fH o rH rH O <M CV| rH CO C^l II II .a r ( ,10 1 o O XCO C^J e^ co o o o o o3 o O O i-J-4 1 1 8 ^t*l^ fbO _JL bn S *^S 5 g^al- g-SSS,**- w q elasticity, Ib. / sq. in. Coefficient of linear ex- * t^ S ^J CD ill "S" opec. rescc Density of Pure Swec CONDUCTORS OTHER THAN COPPER 185 Copper-Cadmium. A copper-cadmium alloy which, has recently been placed on the market seems to be much more promising than bronze, a 35 % increase in strength being secured at the expense of 16 % only in conductivity. For equal conductivity it costs about 30 % more than pure copper (see Table XVIII.). Copper-Steel. Since high tensile steel can be obtained with a breaking stress exceeding 200 000 Ib. / sq. in. compared with 60 000 Ib. / sq. in. lor H.D. copper, it is natural that endeavours should have been made to combine the strength of the former with the conductivity of the latter. A method which suggests itself is to lay up an annular layer of copper wire strands around a steel core, but the objection to this is that the galvanising would quickly disappear by electrochemical action as it is in' contact with copper. This method is used very largely to reinforce aluminium conductors and will be again referred to later. In the U.S.A. a copper-clad steel conductor has been used for many years 3 the copper coating being metallurgically welded on to the steel core, rendering the latter absolutely immune from corrosion. Aluminium. The only other metal in competition with copper from a purely electrical point of view is aluminium. At the prices now ruling, aluminium and copper cost about the same for equal conductivity. It may be said at once, however, that aluminium is not a serious competitor for light overhead lines, owing to the following disad- vantages : (1) Lower Tensile Strength. This necessitates a larger ratio of sag to span length, which in turn means longer and stronger poles. (As a partial set-off, however, the terminal stresses are not so great.) (2) Greater Diameter. This increases the lateral wind loading and therefore somewhat stronger supports are required for this reason as well. (3) Greater Ratio of Wind Load, to Weight of Wire. This neces- sitates greater clearances, as the wires are deflected more from the vertical than copper wires of the same conduc- tivity. This fact often rules out the smaller aluminium wires altogether. (4) More care is required in erecting and jointing. (5) Smaller scrap value. The first three disadvantages can be studied by reference to Tables XIX. and XX. 186 OVERHEAD POWER LINES . coco oo O S7S rldfo co o co o (M in g - - I * v1 '"^ /V1 """* '"'"'* *"* "" **" ^tn 5 55 CMGO'rHairHCONrHCTlOCO _ ~_ rH ro ff. -ft IO O3 CO rH 10 H^ )O I- 1C ' rH ' rH ' O rH 003 -; - "1^'*** ^NCOlpG>l't(OrHt^COi ICO COCOCO eo l> * , i UN rHlOOi IC5i ^O>COrHTHH^O3 Ol ICO ^^CSlCplprHH^OOr ICDOOOCOCOIOO ^q *-~ ' i i I CfJ CD O a-s ^S M o t> rH IO T}< IO t~ ^, ^, ^ ~, ~ ^ t^ ^. ,-, (..J r-, CO ' ' rH Q"3 &.K 'ITTaT-RATnha pj[ispni3().S CO (TO a ._. %j . ^ TO ^ ^ ^ ""-----" CO l> r-H CD IO CO <M CO r-H IO O r-H (N rH . 1 CONDUCTORS OTHER THAN COPPER CO CM os 3 > "-S '"" & H!^ to ^o fi to g KS S ca M II CD S w s- -dug uo 83ugaa|jT(j % CO '? WSt^^ffi -rH Cvl 1O (M =rt t- I-H i~ co (M CO (M N OHM JOd BIO)OllpnoO JO )SOQ O t- t- t- C(-l (M rH rH rH -HO;) ipisgi jo )i{Sio^v '-' 2 3 3 S 3)10 ddng UO 90UQiajJI(I / ^0 1 M (M CO *o|t|\[ jod sQ.ioddng'jo )soo l)ox rH 1O CO 1O trt 10 co 10 -* OHM Md BOIOJ jo -o^i d '= t < -r tx* 1 C3 CO CO i " rH <?l rH CM aloj iod ^SOQ PI^OX t- t^ ob co jo onfBA. posiiu)jd'B3 q^ ip 10 10 in rH rH rH rH pins SinsbO.TQ;' 'sio) V. 9 P 9 '7 Cl rH CI rH Q)iS uo sopd jo iioi^nqujsjci ^ u l -pupti] o\oj jod )soo ^ HH rH C~ rf * 1O CO ^ rH (M rH rH rH rH rH S" * o " ^ CO "^ CO (),ii3ito raoij) op<i jo lo^aofcja: w i a m "3 3 10 10 CO 1> M rH O rH O rH rH rH rH oMJomSuoail^oAo "^ CO CO CO CO puno.io jo quo 4J ^f E~- O CM f^H en i> ob to (M CN W <M dox raofl JO )U10,I JO ODm3)SIC[ 4J rH "^ 1O C3 pLI ... XT ,-l5So' ll3 ,,H .1O1O "*tf -+5 CO CO "^H CO W CO 1O 1O CO CM CM CM CM f f o^^T )^ ^^S 10 1C O ^H & cb 10 10 co pUO'T put M, S g 1 3 r-1 I- S CO 'tio3uQ r T nudg otuiouoog; -xoiddy ?H- g 8 R CM <T1 CO (M (oat -ut-f Suipnp -m) M)aiiii3ici ni'^AQ t- CO CO to rH CO C-. j2 co co co (SOtpUT g ^ rH rH M - 10 co co co a* o o o o M 3213 t <M C4 CM rjt CM CO CO cc t> t- r- g .3 ,11 O <i 00 188 OVERHEAD POWER LINES Steel-Cored Aluminium. To overcome the first disadvantage of ahuninium noted above, steel-cored conductors are used, i.e. conductors made up in the usual way with concentric layers of aluminium wires, but in which the central wire or wires are of high tensile steel. The objection to using a steel core in stranded copper conductors, referred to on page 185, does not appear to hold with aluminium. The explanation is said to be found in the tight bedding of the relatively soft aluminium wires around the core, which, combined with the final filling of any small interstices between the outer aluminium wires with oxide, prevents any penetration of mois- ture inside the conductor. In addition, the fact that zinc and aluminium have a very small electrochemical potential difference undoubtedly has a bearing on the immunity from corrosion. Such a composite conductor is considerably stronger than the electrically equivalent H.D. copper conductor (see Tables XIX. and XX.), therefore the ratio of sag to span length is smaller. This means that longer spans can be used with steel-cored aluminium than with copper and this is of great importance in the transmission of large amounts of power over long distances at very high pressures, which is oubside the scope of this work. It is being used extensively on the 132 000 volt main transmission lines now being erected in this country. It will be noted in Table XX that steel-cored aluminium. shows a saving of 4 to 5 % on the cost of supports and conductors, which means about 2 % on the overall cost of the line. Considering its disadvantages it is not a very attractive proposition for the con- ditions assumed, but it shows far greater economy in long span work. For distribution purposes where comparatively short distances are involved, a very appreciable saving in first cost must be realised to outweigh the difficulties due to jointing and branch connections. This opinion is based on experience with aluminium lines in this country in the last fifteen years, during which, compared with copper, it has not shown up very well. The main difficulty experi- enced has been in maintaining continuity with parallel-groove clamp connections, the contact surfaces of which sooner or later become oxidised, however tightly they are clamped together. However, the aluminium obtainable to-day is much superior in purity to most of that on which the ab.ove opinion is based, and joints undoubtedly give less trouble with purer metal. Also the use CONDUCTORS OTHER THAN COPPER I ^ *= H ^ IN rH CM CO CO l-H iiudg '& ZZ V- ^30^1 001 +i CO rH CO CO CO CO CM !)ooj iad un.r H! 1-7 CO CO rH (N rH 1OOJ Md inij pijoi piiLVV . 00 Hi CO CO o t- CN op aai + O.IIAY ; N M ^ (N CO 9 rH CO w rH nil. i ^ joj iatl a,)y r*l CO CO <N in CS1 OO CO (in |!) un.T ^ooj .wd oaf jo cuiSiaAV . CO ,0 CO Hi CN (N cp ? pcoi SupiioAV ajrcg O ,0 Hi rH o 00 (N CO M < ' Pi Hi o o t- o o o UlU ^00 jod q.tl8i3Ai . CO ,Q t~ H! 9 CO CO 00 000 I .10 (I aouts^otJOii !2 i 1 S t-- <=> IN CO W5 o 000 I J^ aou'Gijsiso'ji 1 *? ^ in rH 10 rH . wn ,o oi M iN i I CO CO . w ,,o .9 * t o en in 9 o rH 'OOUTi^SIS? jo u6( y; c^uaitsAnilig; f oog aaddoo ii (N -i (N O ra 9 C55 o o t- rH pasureAnio jo ozig fe 9 ai ~~~ o rH o rH C- r^gM "5 ^ "p&3 g P S tt o s^ S M ^ -^Sg ^ 03 tS O S "S-a 1 " 1 . r-J **^ .^i .. jy -. . ( S1J1S 1 S CO - ^ 03 !-s| 3* 5; O CO ill'^ 190 OVERHEAD POWER LINES TABLK XXII. Sags and Tensions. Steel Conductors (45 ton Quality) for .Erection Purposes. High Voltage Lines (%-inch ice). 300 7/16 S.W.G. (-064). 7/12 S.W.G. (-104). 7/8 S.W.G. (-16). J 1 Ji'ahfc. RflR, Tension, Sag, Tension, Sag, Tension, loot. Ib. feel;. Ib. foot. Ib. 122 1-52 257 78 1320 05 3700 82 1-04 375 57 1 810 51 4790 02 88 400 50 2000 40 5310 42 74 527 45 2 290 42 5 820 22 04 010 41 2510 38 0430 22 3-32 1080 1-56 2820 92 6 GOO ) 122 5-58 157 1-89 1 230 1-52 3020 82 4-85 181 1-44 1 010 1-19 4020 02 4-50 195 1-28 1810 1-07 5140 42 4-10 214 1-15 2 020 98 5010 22 3-09 238 1-04 2 400 90 0110 22 1-4.11 1080 3-51 2820 2-07 6600 ) 122 11-40 137 4-05 1020 2-80 3 500 82 10-80 144 3-19 1290 2-24 4 370 02 10-45 149 2-83 1400 2-02 4850 42 10-08 155 2-52 1040 1-84 5310 22 9-71 100 2-20 1830 1-08 5820 22 13-28 1080 6-24 2820 3-68 6600 Basic loading sags and maximum legal tensions shown in italics. of " cone " types oi connections wherever possible instead of a clamp will reduce the jointing troubles. Consequently, if there should be an appreciable fall- in the price of aluminium compared with that of copper, the above views may have to be modified. Galvanised Steel. In the above consideration steel has been used really as a carrier for the conductor, but in cases where the BI^O of copper or copper-alloy conductor required from a purely electrical point of view is too small for mechanical reasons it will be sometimes quite feasible to use galvanised steel as the conductor itHolf, in spite of its high resistance. The inductance is naturally greater than that of non-magnetic conductors, but for the small ciirrerrts with which steel is likely to be used the ohmic resistance will CONDUCTORS OTHER THAN COPPER 191 <Sl 1 s : O -S 5 CD g* o 2 1-s s ^> r-* g & I ^ :- ^ ^j ^ <w -s W 55 ^9 f^ o ^ S ,J > O . I14ij[ pur; siop'np B l> s O oo -nog + s^joddng) IM C<1 1 < i i HW wd ^soo r^oi ! i') 8J 8nrc OI S S q?o qj IO IO IO IO 1 (S.IO^t)UpU03) uj CO o o OjTpJ 19(1 ^SOQ rti'|OJj CO IO (raioj) 1Q -^ cs CO o|ij\[ iod 'jsoQ I^ojj ^ N cq 1 1 'I!W -t""! 8 8IcT 1 'Oil . (M cq 5 10 6 CO CO CO t> CO <c o (^ Q 1O "a '010,1 jod aso.") TU'iOT rt ' ' * * CO cq r4 CO t-- t> o Pi H *M OT 'paBIItSJldBQ ert IO IO in IO BGAUOj^ClJjy^ JO !JSO;') A rH rH rH .2 *-w 43^ "UOl^ 3 d -no,ig[ put; SUISSO.TQ; q) l> 00 CO IO CO 5)3 Q) -p '3[ JQAVIIO JJ 0[0 f [ l-H rH r 1 (N M d 'KJO^IUSUI jo ^soo o '(O^TS UO 10 IO d^ nofttiqi-T^sip Suipnp -uj) oioj iod' aoi.1,1 9 P (N CO o CO CO ll r+ ,*. s* co (D =3 O Oi r-l 1 1 rCl 00 +3 O 'OTOfT pIliplITIlQ 1SOJU3 W" ^* ^** ^*^ *""" fc>^ ^ ^ o co -^ CO IO ,O c3 CO -P P a tcl ?o JoijainTSip ^ng[ S 9 ? r"l C33 Oi r-l iM IO t- -ja 'M O rH (1 rH ft^ -P^ poiuiT) p ffi >O ^ IO -ai o[o,j jo wSton " rH N CO CO rH CO CO .a E_i . o o IO ,,, ft tiudg .iod pt)o r i puiAV (J ^ IQ CO CO ft o rH rH o CO ^ IO CO r 1 . T _ ^ r ot' Si>o *j O O ^( CO " IO 1Q <N IO * . O VO O TIl^TT^t ITt'rTo O ^) ft i I i I CO rtt O i-H ft "3 J 1 -" 1 o P-* ^ in en (M <N co cq rH r-f r-l O * '"' ^ r^~ 192 OVERHEAD POWER LINES usually be the predominating factor (see Table XXI). Skin effect is inappreciable. But unfortunately the life of steel is comparatively short and its scrap value is negligible. It quickly rusts when exposed to the atmosphere and must therefore be well galvanised for O.H. line work. The tensile strength of steel increases with the carbon content, but the cost and the specific resistance go up as well ; moreover as the strength increases the steel becomes less ductile and flexible and it rusfcs more quickly. High tensile steel having a breaking stress of 180 000 to 200 000 Ib. / sq. in. is used in the cores of alu- minium-steel cables and is reputed to be immune from corrosion (see p. 188). But it is not usual to employ steel of greater strength than 100 000 Ib. / sq. in. for O.H. power-line conductors, and this will have a shorter life than the standard wires of lower tensile strength used by the post office for telegraph line conductors and stays (B.S. Specs. 182 and 1831923). The latter last upwards of 20 years, except in certain manufacturing areas, whereas the former will probably not last 15 years. Particulars of three sizes of steel conductor are given in Table XXI. and sag and tension values for erection purposes in Table XXII. Table XXIII. illustrates the economy due to the use of these conductors. 193 CHAPTER XI. SAFETY PEECAUTIONS. THE legal regulations on the subject are prescribed by the Electricity Commissioners and the Postmaster-General and they are given in full in Appendices I. to IV. Provisions to Prevent Danger from 1. LEAKAGE. A. Low AND MEDIUM VOLTAGE (Eegulations 13). For pres- sures to earth above 250 volts D.C. or 125 volts A.C. (i) METAL POLES. A continuous earthed wire must be pro- vided, running from pole to pole and connected to all the poles. It must be emphasised that an iron pole does not make a very good earth contact in itself, particularly when, as is frequently the case, it is set in concrete. Hence the danger of touching a pole on which there is a faulty insulator, if a good earth con- nection is not provided. (ii) WOOD POLES. In cases where an earth wire does not form part of the conducting system, all ironwork should be bonded to the earth wire. In cases where there are no earth wires, the ironwork should be bonded together to allow a greater surface for the dissipation of leakage currents. Where the leakage on a line is considerable and the resistance of the earth connections is high, it is possible for the potential of the bonding wires to be raised to a dangerous value. For this reason, if bonding wires or lightning conductors are led down a pole to earth plates it is necessary to insulate the bonding wires to a height of 10 feet from the ground. A cover- ing of creosoted wood casing suffices. STAY WIRES on wood, poles must be considered as 13 194 OVERHEAD POWER LINES part of the metal work and an insulator must be placed in each stay wire at a height of not less than 10 feet from the ground. It should be noted that a neutral conductor must be earthed at one point only, viz. the generating station or sub-station, and it cannot, therefore, be considered as a continuous earthed wire for purposes of complying with this regulation (but see page 54). SERVICE LINES. Special attention is drawn to Regulation 8, which lays down that service lines where accessible must be insulated. To comply with this regulation, all conductors should be covered with durable insulating material within 6 feet of a building. Many fatal accidents have occurred due to neglect of this regulation. B. HIGH VOLTAGE LINES (Regulation 16). In the case of H.V. lines the danger from leakage is obviously greater and all metal work other than the conductor must be bonded together and connected with earth in all cases. Tinned copper or galvanised iron bonding wire should be used. The earthing may be carried out by a con- tinuous wire, earthed 4 times per mile, or, alternatively, the continuous wire may be omitted and the metal work effectively earthed at each pole. The former method is nearly always adopted, since effective earthing is nqt generally practicable at every pole, and although the continuous wire adds consider- ably to the cost of the line it affords a measure of pro- tection against atmospheric effects and it is also useful as a support for auxiliary conductors or telephone cables. The earth connecting wire should be enclosed in creosote wood casing for a distance of 10 feet from ground level. 2. BEOKEN LINE CONDUCTOE. A. Low AND MEDIUM VOLTAGE. The possibility of a con- ductor falling when erected in accordance with the E.G. Regulations must be admitted to be remote, but it has to be considered. E.G. Regulation 13 stipulates the provision of a continuous neutral or other earthed wire SAFETY PRECAUTIONS carried from pole to pole and so arranged as to m contact with a falling conductor. Most L.V. distribution systems work with eart' neutral conductor, which can fortunately be used a continuous earthed wire to comply with this regulati It was formerly the practice to use a so-ca] " split " neutral conductor as illustrated in Figs, and 34 and fix two cross wires in each span as far from the pole as the lineman could reach or alter tively, a single neutral conductor sufficed if a triangu guard was fitted as shown in Kg. 36. But providing the neutral conductor is plai directly below the other conductors, the triangi guard may now be dispensed with, as in Fig. 35. When the line conductors are arranged in triangi fashion as in Fig. 33 and 3d- one earthed conductor i now suffice if it is staggered from side to side at e; succeeding support. As stated before, this methoc construction appears to the writer to present difficult and the vertical arrangement of Figs. 35 and 36 is r likely to be the most popular. B. HIGH VOLTAGE. No special guarding against brol conductors is required, on H.V. lines, except in neighbourhood of roads, railways, etc., but attent may be drawn to the following clause in Eegulation '. " The design and construction of the system of ea connections shall be such that when contact is rcu between a line conductor and metal connected w earth, the resulting leakage current shall not be 1 than twice the leakage current required to operate devices which make the line dead." This implies that ordinary overload protection not sensitive enough for H.V. lines unless three c ditions are satisfied, viz., (i) the neutral of the systen earthed, (ii) suitable guards or earth bars are fit throughout the line so that a broken conductor ^ make contact with earth, and (iii) the overload ti act instantaneously (i.e. no time lag attachments permitted). There are available a number of very sensii 19 6 OVERHEAD POWER LINES protective devices, with wliicli a line conductor on "breaking is made " dead " in a very small fraction of a second, before it has time to reach a person under- neath. But although admittedly the best forms of protection, they entail special line construction with auxiliary conductors and the cost is generally pro- hibitive for minor H.V. distribution lines. For such lines ordinary " leakage " protection will suffice. With earthed neutral at the generating station or transformer station, leakage relays can be relied upon to cut off a line in one-tenth of a second when a leakage current of the order of 5 % of the normal full load current flows, although it may not often be necessary or de- sirable to set the relays too lightly. Additional Safety Precautions in the Neighbourhood of Roads, Railway and Canals. A. L.V. AND M.V. LINES. The regulations for these lines are framed on the assumption that they will be used in residential areas and therefore always in the neigh- bourhood of roads, etc., and except for increased clearances from ground no further precautions are laid down. B. H.V. LINES. (i) CLBAEANGE PROM G-ROUND. In ordinary cross country work, the specified ground clearance of 20 feet is meas- ured from the lowest line conductor, but when the line is erected along or across a public road the clear- ance must be taken from the lowest wire on the pole. This means that the poles must be several feet longer near public roads if earth wires or auxiliary conductors are employed. (ii) WITHIN 50 FEET OP A EOAD OR CANAL. The following alternative safety devices are prescribed : either (a) Duplicate insulators and strap wire ; or (6) Single insulators plus an earthing device. Ref. (a) The duplicate insulators are arranged as shown in Tigs. 26 and 32, and a strap wire of HJD. copper of the same size as the conductor, or, alter- natively, of phosphor bronze of the same strength con- nects the second insulator to points on the conductor SAFETY PRECAUTIONS 197 as far out from the pole on either side as the lineman can reach, i.e. from 3 to 4 feet. This strap wire is intended to prevent the wire from falling to the ground in the event of the line conductor being burnt through at a faulty insulator. That is to say, it is the insulator primarily which causes the anxiety. Ref. (6) An earthing bar or bow of galvanised iron or copper fixed under each conductor is commonly used as shown in Figs. 32 and 37, but the triangular guard arrangement shown in Fig. 36 for L.V. lines is quite suitable on H.V. lines if the conductors are arranged in a vertical plane. The triangular guard can be secured to the earth wire at points 4 feet from the pole and from this point of view it is really better than an earthing bar, which as generally fitted does not usually extend outwards from the pole for more than 18 inches or 2 feet. (iii) CROSSING A ROAD, CANAL OR RAILWAY. The following methods of construction comply with the regulations : Either (a) Duplicate insulators plus duplicate con- ductors ; or (b) Duplicate insulators and strap wire plus earthing device. It will be noted that the regulations make no reservation as to length of span or angle of crossing. It was formerly the practice to shorten up the span at a road crossing as it was believed that a greater factor of safety could be thereby ensured, but it is now gene- rally agreed that it is not good practice to do this. Of 'the two alternative methods, the second is by far the better, as it does not add to the line loading in any way. Among the objections to the first method are the necessary stronger poles with additional line stays, and tensioning insulators on the second conductors. Stranded conductors are preferable to solid ones in the neigh- bourhood of roads. With regard to railway crossings, the railway authorities do not accept the E.G. regulations without qualification and they should always be consulted when surveying a route which crosses their permanent lines. They usually require duplicate conductors with the power line at right angles to the rails and span as short as possible. 198 OVERHEAD POWER LINES Post Office Regulations. In addition to those prescribed by the Electricity Commissioners further regulations laid down by the Postmaster-General must be complied with when power lines are erected in the neighbourhood of signal wires. The P.O. Engin- eering Department's Memoranda on the subject (T.E. 80 and E. in C. 231) are given in full in Appendices III. and IV., but attention is drawn to the following points. A. Low AND MEDIUM VOLTAGE. GUARD WIRES AT CROSSINGS. When practicable the crossing should always be effected with the power conductors above, as being invariably larger wires, erected with a higher factor of safety, they are less likely to fall than the signal wires, and, moreover, it will not be necessary to interrupt the power supply when signal wires are being repaired. Further, the crossing should preferably be as nearly at right angles as possible, as the requirements are then simpler. Independent guard wires may be used in all cases, but if the pressure to earth does not exceed 250 A.C. and the signal wires are underneath, it will usually be found more convenient and economical to split the neutral of the power system at the crossing span, cross- lacing every 6 feet as required. If the signal wires are above, it will be necessary to run an ad- ditional earthed wire above the power wires, unless the arrangement shown in Tig. 34 is adopted with split neutral and cross lacing. The normal specified clearance between guard wires and signal wires is 4 feet, but in special cases a clearance of 2 feet may be ac- cepted. It will be noted that no guard wires are required when the power conductors are insulated and supported by an earthed bare suspen- sion wire, nor are they required up to 250 volts A.C. if either the power conductors or the signal wires are insulated with an approved weather- proof covering (see Specification, p. 232). HIGH VOLTAGE. The P.O. still prefer (but do not now insist) that either the power wires or the signal wires should be placed underground at crossings. The requirements are roughly indicated in Eig. 92. In special cases, however, the department has agreed to smaller values of x and a', but this has never been less than 1-| G f , the height of the signal route, or 10, the height of the power circuit route, whichever is the greater. This allows a smaller clearance than the SAFETY PRECAUTIONS 199 standard rule in cases where the power wires are higher than the signal wires. But apart from the question of cost there are important electrical objections to inserting short lengths of cable in either line. This is now more fully realised than formerly and the problem became acute with the advent of 33 000 volt and 66 000 volt transmission. The latest P.O. regulations, therefore, permit overhead crossings with duplicate conductors plus a cradle guard, which is a compromise to b orC'^Ju'cherei* is the qr-eoter) + d (difference of fetv/J FIG. 92. that may cost as much as a cable crossing, but is more satisfactory from an electrical point of view. It may also be noted that consent has been obtained to suspend an armoured cable from a steel wire, which costs much less than putting it underground, because the use of terminal poles and stays is avoided, the line tension being carried through by the suspension In this connection it may be noted that up to 6 600 volts, wire- armoured, ozone-proof cable is better than lead-covered paper cable, as it is lighter and the cost of sealing boxes is avoided. 301 APPENDIX I. ELECTBICITY (SUPPLY) ACTS, 1882 TO 1926. [El. C. 53.] OVERHEAD LINE REGULATIONS for securing the Safety of the Public made by the Electricity Commissioners under the Electricity (Supply] Acts, 1882 to 1926. DEFINITION. In the following Regulations the expression " line conductors " means conductors used for transmitting a supply of electrical energy, including so much of any service line as may be under the control of the undertakers. I. GENEBAL. Material of Line Conductors. 1 . Line conductors shall be of copper, aluminium, or such other materials as may be approved by the Electricity Commissioners. Strength of Line Conductors. 2. All line conductors at the time of erection shall comply, as regards elongation, breaking load and elasticity, with the specifica- tion of the British Engineering Standards Association then in force. Minimum Size of Line Conductor. 3. The minimum permissible size for copper and other line conductors (other than service lines) shall be such as to have an actual breaking load of not less than 1237 pounds, the equivalent minimum cross-sectional area and weight per mile for copper being as follows : Conductor. Cross-sectional area, Weight per Mile, sq.. ins. Ibs. No. SS.W.a . 0-0201 409 202 OVERHEAD POWER LINES The minimum permissible size of service line shall be such, as to have an actual breaking load of not less than 816 pounds, the equiva- lent minimum cross-sectional area and weight per mile for copper being as follows : Conductor. Cross-sectional area, Weight per Mile, sq. ins. Ibs. No. 10 S.W.G. 0-0129 262 Line Conductors to be inaccessible. 4. Line conductors shall be rendered inaccessible to any person from any building or other place without the use of a ladder or other special appliance. Regard shall be had to the normal use by the occupier of any premises or land and where necessary (a) the height of the line conductors shall be increased to provide sufficient clearance for safety in accordance with such use, and (6) provision as hereinafter prescribed in Regulations 14 or 17 shall be made to prevent danger. Line Conductors Crossing other Lines. 5. "Where a line conductor crosses over or under, or is in proximity to any other overhead wire, precautions shall be taken by the undertakers to prevent contact, due to breakage or otherwise, between the line conductor and the other overhead wire, or between the other wire and the line conductor. Provided that this Regulation shall not be deemed to require the undertakers to take precautions against contact between a broken line conductor and other auxiliary conductors and earth wires carried on the same support and forming part of the same overhead line. Supports. 6. Line conductors shall be attached to suitable insulators carried on supports of wood, iron, steel or reinforced concrete. All wooden supports other than oak or hard wood cross-arms shall unless other- wise approved by the Electricity Commissioners be of red fir impreg- nated with creosote. Special precautions shall be taken to prevent the corrosion of all metal work at or below the surface of the ground. Factor of Safety of Supports. 7. The supports, in conjunction with stays or struts if provided, shah 1 withstand the longitudinal, transverse and vertical loads due *" ^e ice loadings and wind pressure hereinafter specified without ge and without movement in the ground. In no case shall APPENDIX I 203 the strength of a support in the direction of the overhead line be less than one-quarter the required strength in a direction transverse to the line. The following factors of safety shall apply to each support : Material. Factor of Safety. Iron or steel . . . 2-5 Wood .... 3-5 Reinforced concrete . . 3-5 These factors of safety shall be calculated on the assumption that all line conductors cables and wires carried by the supports are at a temperature of 22 E., and have a covering of ice to the radial thickness specified in Regulation 12 (or Regulation 15 according to the voltage) and that together with the supports they are subjected to a, wind of 50 miles per hour at right angles to the line, this wind to be taken as exerting a pressure equivalent to 8 pounds per square foot calculated on the whole of the projected area. The wind pressure on the lee side members of lattice steel or other compound structures, including A and H poles, shall be taken as one-half of the wind pressure on the windward side members. The factor of safety shall be calculated on the crippling load of struts and upon the elastic limit of tension members. Service Lines. 8. Service lines shall be connected to line conductors at a point of support only and shall be fixed to insulators on consumers' premises. Every part of a service line (other than a neutral con- ductor connected with earth) which is accessible from a building with the use of a ladder or other special appliance shall be efficiently protected either by insulating material or by other means approved by the Electricity Commissioners. Erection of Line Conductors at Different Voltages on same Supports. 9. Where line conductors forming parts of systems at different voltages are erected on the same poles or supports adequate pro- vision shall be made to guard against danger to linesmen and from the lower voltage system being charged above its normal voltage by leakage from or contact with the higher voltage system ; and the type of construction shall be subject to the prior approval of the Electricity Commissioners. 204 OVERHEAD POWER LINES Inspection and Maintenance of Lines. 10. Every overhead line, including its supports and structural parts, and electrical appliances and devices belonging to or con- nected therewith, shall be regularly inspected and efficiently main- tained. Materials used, 11. All materials used shall at the time of erection conform to the specifications of the British Engineering Standards Association and to the Post Office Technical Instructions for the construction of aerial lines for the time being in force, so far as the same are applicable and are not inconsistent with these Regulations. II. SPECIFIC REGULATIONS. [Applicable according to the voltage between line conductors where no part of the system is connected with earth, or according to the voltage to earth where part of the system is connected with earth,] A. For voltages not exceeding 650 volts direct current and 325 volts alternating current. Factor of Safety of Line Conductors. 12. The factor of safety of line conductors shall be 2. The factor of safety shall be based on the breaking load and shall be calculated on the assumption that the line conductors are at a tem- perature of 22 F. and have a covering of ice to a radial thickness of three-sixteenths of an inch, and that they are simultaneously subjected to a wind of 50 miles per hour at right angles to the line, this wind to be taken as exerting a pressure equivalent to 8 pounds per square foot calculated on the whole of the projected area of the ice-covered lines. The weight of ice is to be taken as 57 pounds per cubic foot. The elasticity of the metal may be allowed for in calculating the sag for line conductors. Minimum Height of Conductors. 13. The height from the ground of any line conductor (other than a service line), earth wire, or auxiliary conductor at any point of the span -at a temperature of 122 E. shall not, except with the consent of the Electricity Commissioners, be less than 19 feet across a public road or 17 feet in other positions. A height of 15 feet may be adopted in situations inaccessible to vehicular traffic. APPENDIX I Where a service line is carried across or along a carriag* the height of the line from the ground at any part of the cai way shall not, except with the consent of the Electricity Co: sioners, be less than 19 feet and 17 feet respectively. Provision to Prevent Danger. 14. Where the voltage to earth exceeds 250 volts direct ci or 125 volts alternating current, precautions should be tat prevent danger (1) from a broken line conductor by the provision of (a) a neutral or earthed conductor carried contini from pole to pole, and so arranged in relation other conductors that in the event of breakage < one of them the line conductor shall make c< with the earthed wire ; or (6) other means approved by the Electricity Commissi (2) from leakage by the provision (a) in cases where metal poles are used, of (i) an earthed wire, running from pole to pole an< nected to the poles ; or (ii) a suitable metal framework to support the insi carrying the line conductors, the framework insulated from the pole but connected to the n conductor ; or (iii) other means approved by the Electricity Co sioners. (6) in cases where wooden poles are used, of (i) a bonding wire connected to the supporting : work of all insulators, the bonding wire termi: at the lowest part of the supporting metal or (ii) other means approved by the Electricity Co sioners. Where lightning conductors are used or other uninsulate< ductors are run down wooden poles to within 10 feet fro ground, the precautions for the prevention of danger from 1< shall be as for metal poles. All stay wires other than those which are connected witl by means of a continuous earth wire shall be insulated to p danger from leakage. For this purpose an insulator shall be in each stay wire at a height of not less than 10 feet from the g B. For voltages exceeding 650 volts direct current and 32 alternating current. 206 OVERHEAD POWER LINES Factor of Safety of Line Conductors. 15. The factor of safety of line conductors snail be 2. The factor of safety shall be based on the breaking load and shall be calculated on the assumption that the line conductors are at a temperature of 22 E., and have a covering of ice to a radial thickness of three- eiglths of an inch, and that they are simultaneously subjected to a wind of 50 miles per hour at right angles to the line, this wind to be taken as exerting a pressure equivalent to 8 pounds per square foot calculated on the whole of the projected area of the ice-covered lines. The weight of ice is to be taken as 57 pounds per cubic foot. The elasticity of the metal may be allowed for in calculating the sag for line conductors. Minimum Height of Gondiictors. 16. The height from the ground of any line conductor at any point on the span at a temperature of 122 F. shall not, except with the consent of the Electricity Commissioners, be less than the height hereunder stated : Voltages not exceeding\ 9n , , Voltages exceeding"] 66 000 volts. ' / U 110 000 volts and not I ^ f Voltages exceeding^ exceeding 165 000 j 66 000 volts and not 1 9 , , , volts. J exceeding 110 000 I ^ el " Voltages exceeding^ , volts. ^ J 165 000 volts. j^teet. The height from the ground of an earth, wire or auxiliary con- ductor shall not be less than the minimum heights prescribed in Regulation 13. Provision to Prevent Danger. 17. Adequate means shall be provided to render any line con- ductor dead in the event of it falling due to breakage or otherwise. All metal work other than conductors shall be permanently and efficiently connected with earth. Eor this purpose a continuous earth, wire shall be provided and connected with, earth at four points in every mile, the spacing between the points being as nearly equidis- tant as possible, or, alternatively, the metal work shall be connected to an effective earthing device at each individual support. The design and construction of the system of earth connections shall be such that when contact is made between, a line conductor and metal connected with earth the resulting leakage current shall not be less than twice the leakage current required to operate the devices which, make the line dead. APPENDIX I Road Crossings, cfec. 18. Where an overhead line is erected along or across a pub road or canal or across a railway all wires including earth, wires ai auxiliary conductors shall be placed at the appropriate height frc the ground specified in Regulation 16 for line conductors, and t following additional precautions shall be taken to prevent danger : (1) In the case of a line erected along a public road or canal ( within 50 feet thereof) there shall be provided (a) duplicate insulators supporting the conductors ; or (6) a device to ensure that in the event of a line conduct falling it shall be put to earth ; or (c) other means approved by the Electricity Commissione: (2) In the case of a line erected across a public road, canal railway there shall be provided (a) duplicate insulators for supporting the line conductor a a device to ensure that in the event of a line cc ductor falling it shall be put to earth ; or (b) duplicate insulators supporting duplicate conductors ti at intervals not exceeding five feet ; or (c) other means approved by the Electricity Commissione Danger Notices. 19. Supports shall be numbered consecutively and each suppi shall have a danger notice of a permanent character securely 'fix to it. Adequate provision shall also be made to prevent unauth< ised climbing. These Regulations are made subject to the power of the EL tricity Commissioners to make such further or other Regulatic as they may think expedient and shall apply to any overhead Hi erected by authorised undertakers : Provided that these [Regulations shall not apply to any overhe lines in existence at the date hereof and constructed and maintain -by authorised undertakers under and in accordance with the p: visions of any prior Regulations for overhead lines made by 1 Board of Trade or -the Electricity Commissioners. Signed by order of the Electricity Commissioners this 16th d of April, 1928. R. T, Gk B 1 BENCH, Secretary to the Electricity Commission* 208 OVERHEAD POWER LINES ELECTRICITY COMMISSION. EXPLANATORY MEMORANDUM on the Revised Code of Overhead Line Regulations (El. C. 53A) made by the Electricity Commissioners and on matters relevant thereto. The Electricity Commissioners have recently reviewed the Code of Overhead Line Regulations (El. C. 39) adopted by them in the latfcer part of 1923, with the object of determining whether relaxa- tions consistent with the maintenance of a reasonable degree of safety to the public could be made for the purpose more particularly of facilitating the development of overhead distribution at the lower voltages in rural areas. Due regard has been given to various representations on this matter which have been made to the Commissioners from time to time on behalf of the Electricity Supply Industry, and the technical aspects have formed the subject of conferences between the Com- missioners and Electrical Engineers who have had considerable experience in the erection of overhead lines in rural districts and elsewhere. The Commissioners have also conferred with the Post- master-General on the question of the relaxation of his requirements as to protection between telegraph or telephone lines and electric supply lines. As a result, the Commissioners have adopted a revised Code of Overhead Line Regulations (EL C. 53) with effect as from 16th April, 1928, and the following explanatory notes dealing with the various relaxations and with other relevant questions have been prepared for the information of the Electricity Supply Industry. REGULATION 1 (Materials for Line Conductors). This Regulation remains unaltered, but it may be noted that the " other materials " for line conductors which have already been approved by the Electricity Commissioners in certain cases include steel-cored aluminium, steel, cadmium copper and other appropriate alloys of high tensile strength. REGULATION 3 (Minimum Size of Line Conductor). In view of the relaxation which has been made in the assumed ice loading on line conductors (Regulations 12 and 15) and in guard- ing requirements (Regulation 14), the minimum size of copper con- ductor (other than service lines) is in future to be No. 8 S.W.G-. or the nearest equivalent section of stranded conductor (being not less than. APPENDIX I 209 No. 8 S.W.G.). The minimum size of lines in general (other than service lines) is to be such that the actual breaking load shall be not less than 1 237 Ib. REGULATION 6 (Supports}. Under this Regulation, as amended, the Commissioners will be prepared in special cases to consent to the use of wooden poles other than of red fir, provided that suitable precautions are taken in the felling, selection and treatment of the timber. REGULATION 7 (Factor of Safety of Supports). After consultation with the Air Ministry, the National Physical Laboratory and the British Electrical and Allied Industries Research Association on the questions of climatic conditions, wind velocities and wind pressures, the Commissioners have decided to retain the same wind pressure and the same factors of safety as are prescribed 'by the existing Code in respect of supports and also of line conductors. The wind pressure on the supports alone has been shown to vary with different types of support, but is usually only a small proportion of the total load on the supports. The Commissioners have concluded, however, that the relaxed conditions with regard to the assumed ice loading on line conductors (Regulations 12 and 15) will enable supports of suitable size and strength, but of less expensive construction than hitherto, to be adopted consistently with securing the safety of the public. An enquiry is being continued into the factor of safety of reinforced concrete supports. With regard to the foundation of supports, the Regulation now requires that the supports shall withstand the specified ice loadings and wind pressure without movement in the ground. As the working load under the Revised Code will constitute a greater proportion of the ultimate strength of the supports than under the prior Code, it will be more particularly necessary to pay attention to the founda- tions of wooden poles, which will require the addition of cross mem- bers or " kicking blocks " (excepting in the case of poles of small sizes) in order that the foundation may be as strong as the pole itself. REGULATION 8 (Service Lines}. The requirement for the protection of persons working on the outside of buildings may be generally assumed to involve the cover- ing of all live conductors with durable insulating material for so much of their length as lies within 6 feet of the building. 14 210 OVERHEAD POWER LINES REGULATION 9 (Erection of line Conductors at Different Voltages on same Supports}. Although the construction of lines at different voltages on the same supports was not precluded under the prior Code, the Com- missioners have considered it desirable to include a new Regulation dealing specifically with this matter. The Commissioners will be prepared to approve of types of construction under this Regulation which make adequate provision for avoiding danger to linesmen working on the lines and for preventing contact between the higher and lower voltage systems. REGULATION 12 (Factor of Safety of Line Conductors}. The Commissioners have made a relaxation in the assumed ice loading on lower voltage lines from one-quarter of an inch to three- sixteenths of an inch, but, as previously indicated, have retained the provisions of the prior Code as to wind pressure and factor of safety. The assumed ice loading and unaltered factor of safety will permit of line conductors being strung to a tension which is at least as high as may profitably be used and is consistent with the tension when the line withoutthe assumed ice loading is subjected to a wind of gale force equivalent to a pressure of 20 Ib. per sq. foot. REGULATION 13 (Minimum Height of Conductors}. The minimum height from the ground of lower voltage line con- ductors has been reduced from 20 feet to 19 feet across public roads ; to 17 feet along public roads and in other positions ; and to 15 feet in situations inaccessible to vehicular traffic. The minunum height of service lines has been reduced from 20 feet to 19 feet across carriage-ways and to 17 feet along carriage- ways. REGULATION 14 (Provision to Prevent Danger}. Provided that one of the conductors is properly connected to earth at the point of supply and that the lines are so arranged that the earthed conductor is placed below the other conductors, no other guarding on lower voltage lines will be required. If the conductors are arranged in a vertical plane with the earthed conductor placed lowermost, the protection afforded is adequate. If the conductors are not so arranged, then the earthed conductor, which must still be erected in the lowermost position, should be staggered from right to left of each succeeding support so as to afford APPENDIX I a reasonable certainty that a broken conductor will make contact with the earthed conductor. With the reduction in the minimum height from the ground afforded by regulation 13, many lower voltage lines will be a,ble to pass beneath other lines such as telegraph or telephone wires. The Commissioners are in a position to state that in such cases the re- quirements of the Postmaster-General for guard wires will be satisfied if a single earthed wire is run above the power wires for the length of the span. The Postmaster-C4eneral has further agreed to a relaxation in the Post Office Memorandum T.B. 80 (protection of telegraphs from contact with low and medium pressure power circuits) by modifying his present requirement of 4 feet clearance between low and medium pressure power wires and telegraph lines to the extent of allowing a minimum of 2 feet if the power line supports are placed in such a position with, respect to Post Office wires that there shall be no danger to men working on the Post Office poles and that there shall be no appreciable difference in sag under the worst conditions of ice loading. The requirement of a bonding wire connected to the supporting metal work of all insulators used with wooden poles appears to be necessary in the interest of authorised undertakers ; and a suitable earthing of metal poles is likewise necessary. These features are therefore retained in the new Regulations. Where lightning conductors are used with wooden poles, it is desirable that to a height of 10 feet from the ground they should either be insulated or connected to a continuous earth wire having an efficient earth connection. The Commissioners have also con- cluded that where stay wires are used in conjunction with wooden poles, it is necessary that insulators should be interposed in the stay wire. Such insulators would not be necessary in stay wires fixed to metal poles which are protected from leakage in accordance with Regulation 14 (2). REGULATION 15 (Factor of Safety of Line Conductors). The Commissioners have made a reduction in the assumed ice loading on higher voltage lines from one-half of an inch to three- eighths of an inch, but, as previously indicated, have retained the provisions of the prior Code as to wind pressure and factor of safety. The relaxation affords a reasonable measure of relief from the original requirement. OVERHEAD POWER LINES REGULATION 16 (Minimum Height of Conductors}. In the case of higher voltage line conductors, the Commissioners have decided to retain the provisions of the prior Code as to minimum, height above ground. The Regulation has been amplified, however, to deal with the minimum height of associated earth wires or auxiliary conductors. General Observations. The combined effect of the preceding relaxations in the case of lower voltage lines will be found to be such as to enable supports of reduced diameter as well as of reduced length to be employed. For example, making due allowance for the reduction in the height of a lower voltage line above the ground from 20 feet to 17 feet (Regulation 13), a wooden pole having a diameter of only 7J inches at a distance of 5 feet from the butt will hereafter be needed where previously a diameter of 8J- inches would have been required under the prior Code. As a further means of facilitating the extension of overhead distribution in rural areas, the Commissioners are prepared, under their Regulations for securing the safety of the public and for en- suring a proper and sufficient supply of electrical energy, to give consent in special cases to a voltage variation within the limits of plus 4 and minus 8 per cent, of the declared voltage on rural lines for a provisional period pending the completion of the distribution system and its full measure of interconnection. After such pro- visional period, the variation can reasonably be limited to the normal requirement of plus or minus 4 per cent. Procedure. The Commissioners suggest that authorised undertakers should take a reasonably long view of the probable development of low voltage overhead distribution in their area and should, where practicable, make application to the Minister of Transport for a consent to more comprehensive proposals than hitherto instead of making repeated applications for individual lines, thus saving time and expense. With the view of expediting the consideration of applications, the procedure formerly adopted by the Board of Trade and con- tinued by the Commissioners on behalf of the Minister of Transport, of referring all overhead line applications to the Postmaster-General for his observations will, by agreement between the Departments, "be abandoned as from the issue of the Revised Regulations. The APPENDIX I 213 Commissioners suggest, however, that in all cases where it is in- tended to run high voltage or extra high voltage overhead lines for an appreciable distance parallel with Post Office overhead routes, the authorised undertakers concerned should adopt the general practice of conferring with the Post Office engineers in their district prior to applying for consent to such overhead lines, so that the possibility of inductive interference may be considered before the route is actually fixed. It will, of course, still be necessary for the undertakers to give statutory notice to the Postmaster-General in compliance with Section 14 of the Schedule to the Electric Lighting (Clauses) Act, 1899. With the concurrence of the Minister of Transport, the existing Memorandum of particulars required in connection with proposals to erect overhead lines (Form El. C. 34) has been modified with the view of simplifying procedure. The main alterations in the Revised Memorandum (El. C. 53B) are as follows : (a) Legible tracings or prints taken from 6-inch Ordnance maps may now be submitted in lieu of the maps themselves ; and in cases relating to overhead lines at extra high voltage, duplicate maps will not in future be required. (b) Where an undertaker has submitted full technical details with one application and consent is subsequently sought to the construction of further overhead lines to the same specification, it will not be necessary to resubmit full technical details, but only to furnish certain limited particulars. (c) A form of communication which the undertakers should send to the Local Authority or County Council when proposing to apply to the Minister for consent to erect overhead lines has been drawn up and is included in the Revised Memorandum. Electricity Commissionj Savoy Court, Strand, W.C. 2, April, 1928. 214 APPENDIX II. ELECTRICITY (SUPPLY) ACTS, 1882 TO 1926. [El. C. 53e.] OVERHEAD LINES. MEMOEANDUM setting forth the information to be submitted in connection with applications by Authorised Undertakers for the consent of the Minister of Transport to the placing of electric lines above ground. 1. An application, for the consent of the Minister of Transport to the placing of an electric line above ground should be formally addressed to The Secretary, Ministry of Transport, Whitehall Gardens, S.W. 1, but may be delivered direct to the Office of the Electricity Commissioners who advise the Minister in connection with the technical aspects of all overhead line proposals. 2. The application must be accompanied by the technical and other particulars set out in the Schedule appended to this Memoran- dum duly signed on behalf of the undertakers. 3. Where the electric lines forming the subject of any application are to be constructed in accordance with details already submitted with a prior application to which the consent of the Minister has been given, it will only be necessary for the undertakers to submit certain details as indicated in the Schedule and to complete and sign the Certificate at the end of the Schedule. 4. The undertakers should serve a notice of their application as nearly as may be in the form set out in the next page of this Memo- randum, together with a description of the nature and position of the proposed lines, on the local authorities in whose districts the lines are to be placed and on the County Council in cases where a county bridge or a main road vested in such Council is concerned. The local authorities in England and Wales are Borough Councils, Urban District Councils and Rural District Councils. The local authorities in Scotland are Police Commissioners, Gas Commis- sioners, Town Councils and County Councils. 5. In making application to the Minister, the undertakers should give the date of the service of the above-mentioned notice, and a list of the authorities upon whom the notice has been served. Where APPENDIX II 215 possible^ the undertakers should submit evidence showing whether or not the authorities desire to be heard by the Minister. 6. Where it is proposed to place an electric line across any land (other than a street or public bridge) or across or along any railway, canal, inland navigation, dock or harbour, the undertakers should state whether wayleaves have been agreed with- the owner and oc- cupier of the land or with the owners of the railway, canal, inland navigation, dock or harbour as the case may be. 7. Attention is drawn to the revised Code of Overhead Line Regulations of the Electricity Commissioners (Form EL C. 53) and to the Explanatory Memorandum (El. 0. 53A) issued in connection therewith ; and to the provisions relating to tlie approval of plans and works contained in Section 14 of the Schedule to the Electric Lighting (Clauses) Act, 1899, or corresponding provision in the Undertakers' Act or Order. * Ministry of Transport, 6 Whitehall Gardens, S.W.I, ^7, 1928. Form of Notice. ELECTEICITY (SUPPLY) ACTS, 1882 TO 1926. (Title of Order or Special Act.) Sir, I beg to inform you that the (insert name of applicants) have made an application to the Minister of Transport for consent to the placing of electric lines above ground for the purposes of the above- mentioned Order (or Special Act), and in this connection desire to draw the attention of your Council to the provisions of Section 21 of the Electricity (Supply) Act, 1919, as amended by Section 50 of and the Sixth Schedule to the Electricity (Supply) Act, 1926. The Acts in question provide that where the consent of the Minister of Transport is obtained to the placing of any electric line above ground in any case, the consent of the local authority (in- cluding a County Council) shall not be required, anything in the Electric Lighting Acts or in any Order or Special Act relating to the undertaking to the contrary notwithstanding, but the Minister ^ of Transport before giving his consent shall give the local authority and (where it is proposed to place the line along or across any county bridge or any main road vested in a County Council) the County Council an opportunity of being heard. A description of the nature and position of the proposed lines so far as they affect your Council is enclosed herewith and I shall be 216 OVERHEAD POWER LINES glad to learn at your earliest convenience whether (insert name of applicants) may notify the Minister that your Council do not desire to be heard in connection with the application. (To be signed on behalf of the Applicants.) NOTE. " ; "- regarc tikis Schedule. Schedule of Particulars. ; - - - ' -* items (l)to(l}(d}are required in every case. With . attention is drawn to the Certificate at the end of (1) An Ordnance map on a scale of six inches to the mile (or a tracing or print therefrom giving appropriate reference to the Ordnance map) must be supplied showing : (a) The proposed route of the line with positions of the terminal, intermediate and angle supports, and of the "earth "plates. Any underground, portion of the line should be shown in distinctive colour. (b) Any existing overhead lines, whether for power, lighting, traction, telegraph or telephone purposes, in the immediate vicinity of the proposed trans- mission lines. (2) Working voltage Volts. (3) Is supply by direct or alternating cur- rent ? (4) If by alternating current, state number of phases and frequency . Phase. Cycles. (5) Maximum amount of energy which the line is designed to transmit, in kilo- watts ...... (6) Total route length of overhead line, in yards ...... (7) Conductors : (a) Number ..... (b) Material used .... (c) Solid or Stranded .... (d) Sectional area of each conductor and when stranded, the number and diameters of wires *(o) Height of lowest conductor (or earth or neutral wire if forming part of the conducting system) above ground at pole, in feet .... *(f ) Sag of lowest conductor (or earth or neutral wire if forming part of the conducting system) at temperature of 122 Fahrenheit on maximum span, in feet .... *(g) Minimum height above ground of lowest conductor or wire between poles on maximum span, in feet . *(h) Breaking load of materials in tons per square inch .... *(i) Elongation of conductor in length of 10 inches on breaking, per cent. . (a), (b), (c).. (d)., (e). (f). (g)- (h).. APPENDIX II *(8) Earth Wire (not forming part of the con- ducting system) or auxiliary con- ductor : (a) Size ...... (a) (b) Description ..... (b) (c) Height above ground at pole, in feet (c) *(9) Span between poles or other supports : (a) Average span, in feet (a) (b) Maximum span, in feet (b) (10) Poles : (a) Class of pole to be used, i.e. Wood, Steel Tubular, Lattice or other ma- terial. See paragraph (13) below (a) (b) Diameter of pole at top, in inches b) (c) Diameter of polo at 5 feet from butt in inches .... (c) (d) Depth of pole in ground, in feet (d) (e) Overall length of pole, in feet (e) (f ) If wooden polos are used, the nature of the timber .... (f) (g) If steel tubular poles are used, the thickness of metal, in inches (g) (h) Breaking stress of steel used, in poles, in tons per square inch (h) *(11) Type of automatic protective device . *(12) Earth plates : (a) Tvpe (a) (b) Dimensions ..... fb) (c) Metal ..... (c) (d) Number proposed (d) *(13) A drawing (scale to be stated and to be not loss than one-half inch to the foot) of each type of pole proposed to be used, with dimensions as in Fig., must bo supplied showing : (a) Details of stays and struts. (b) Cross-arms. (c) Insulators. (d) Arrangement of conductors and their sizes indicated against each insulator, (o) Safety arrangements at road, railway and canal crossings. (f) Earth wire and earth plates. Tf steel lattice masts, reinforced concrete poles or supports of special design are proposed to be used, stress diagrams with detailed calculations must be submitted in addition to the drawing referred to above. The particulars set out against items of the above Schedule relate to tho overhead lines forming the subject of the application made on by the (Signed Electrical Engineer to the Undertakers. Diameter I '//y//// T ,T Diameter 218 OVERHEAD POWER LINES * Certificate. (In cases where details (7) (e) to (13) submitted with a prior applica- tion are applicable.) I HEREBY CERTIFY that the overhead lines forming the subject of this application will, so far as items (7) (e) to (13) of the pre- ceding Schedule are concerned, be constructed in accordance with the details submitted in connection with a prior application made on to which the consent of the Minister of Transport was given on (Signed) Electrical Engineer to the Undertakers. 219 APPENDIX III. POST OFFICE ENGINEERING DEPARTMENT. [E. in C. 231.] MEMORANDUM on Protection of Overhead Telegraph or Telephone wires at Crossings of High or Extra High Pressure overhead Power Linen. THE Electricity Commissioners' Regulations for Overhead Power Lines stipulate that where a line conductor crosses over or under or is in proximity to any other overhead wire, precautions shall be taken by the undertakers to prevent contact, due to breakage or otherwise, between the line conductor and the other overhead wire, or between the other wire and the line conductor. "Where the power circuit is classified as high or extra high pressure it is the practice to arrange for either the power wires or the tele- graph and telephone wires to be placed underground at crossings. Where the Post Office circuits are local and there is little loss in efficiency by placing them underground this method of protection will be adopted (otherwise the power line should be placed under- ground for the requisite distance). In cases where there are serious objections to either the telegraph or the power line being placed underground the method shown in the annexed sketch will be adopted, the protection taking the form of a substantial cradle guard. The power line and the guard should comply with the following conditions : (1) The routes must cross at right angles and continue at right angles for a distance of not less than 20 yards on each side of the crossing. In difficult cases, however, a deviation up to 30 degrees from the right angle will be accepted for a straight- through crossing. (2) The power lines must cross above the telegraphs or tele- phones. (3) Duplicate conductors lashed every 5 feet and terminated on separate insulators must be provided for each power wire at each end of the crossing span ; alternatively a single conductor will be accepted, provided that it is stranded and used with duplicate insulators, bridles, and the earthing device specified by the Electricity Commissioners : (4) The poles oF structures supporting the power wires at the crossing span must be sufficiently strong to serve as terminals should the wires in adjacent spans break. 220 OVERHEAD POWER LINES Q: APPENDIX III The deflection of the structures under such, conditions must be less than would permit the power conductors to sag on the guard. The poles should preferably be of steel or iron but if of wood all metal fittings shall be connected to earth by low resistance conductors as specified for the guard under condition (5). (5) The independent poles supporting the cradle guard may be of wood, iron, steel or reinforced concrete. The clearance between guard and power conductors must be sufficient to pre- vent contact under the worst possible conditions, otherwise than by breakages of conductors. The top or outside wire on each side of the independent cradle guard shall be so arranged that lines drawn upwards from them towards the centre at an angle of 45 degrees will totally enclose the power wires together with any telephone control or other wire belonging to the power system. The guard to be made of wire of not less than 7/14 S.W.G. galvanised steel or hard drawn copper. The cradle will be cross-laced every two feet above the tele- graph wires to a distance of 6 feet beyond the wires on each side. The guard to be connected to earth at each end of the crossing. The resistance of guard to earth shall not exceed 1 ohm or 60/A ohms, whichever is the smaller value. (A = max. current of system.) The earth and earth connections must be capable of carrying the maximum current which can flow to earth in the event of a contact between a power conductor and the guard. In the case of wood poles, the earth connection shall be so grooved into the pole that there will be no danger of the wire being tampered with. (6) The clearance between the guard and the telegraphs must not be less than 3 feet. (7) The poles, structures and wires of the power lines must be constructed with the factors of safety laid down in the Elec- tricity Commissioners' Regulations for Overhead Lines. (8) The structure supporting the power line crossing span to be placed in positions free from, risk of damage by traffic. (9) Alternatively to the provision of an independent guard, the erection of a cradle guard on the power circuit supports will be accepted, providing the structures are of steel or iron, very stable in design and that other conditions are satisfactory. (10) Detailed drawings and plan to be submitted to Bngineer- in-Chief, Post Office, for approval, the erection and maintenance to be to the satisfaction of the Engineer-in-Chief. APPENDIX IV. POST OFFICE ENGINEERING DEPARTMENT. [T.E. 80.] MEMORANDUM: on Protection of Telegraphs from Contact with Low and Medium Pressure Power Circuits (excluding Traction Circuits). 1. The Electricity Commissioners' Regulations for Overhead Power Lines stipulate that where a line conductor crosses over or under or is in proximity to any other overhead wire, precautions shall be taken by the undertakers to prevent contact, due to breakage or otherwise, between the line conductor and the other overhead, wire, or between the other wire and the line conductor. 2. For the purposes of this Memorandum the expression " tele- graphs " includes all telegraph and telephone conductors and also stay wires; and the expression "power circuit" means any con- tinuous current * or alternating current * power circuit (other than a traction circuit) so arranged that the maximum pressure between any two conductors of a circuit entirely insulated from earth or between any conductor and earth, in the case of a circuit earthed at the power station, sub-station or transformer, does not exceed 650 volts. 3. When the pressure to earth, or between any two conductors of an unearthed system, does not exceed 60 volts A.C. or 120 volts C.C., no guarding is required. 4. When the pressure to earth, or between any two conductors of an unearthed system, exceeds 60 volts A.C. or 120 volts C.C., guarding is required : (a) at each point of crossing or overhanging ; (6) in the case of parallel lines, where the vertical distance between any telegraph, and any power wire exceeds the horizontal distance | (see Pig. 1), and the approved arrangement and the scope of each are as follows : * In further references to " continuous current " and " alternating current " the abbreviations " C.C." and " A.C." respectively are used. f This corresponds to the 45 rule applied to electric tramway and trackless trolley systems. APPENDIX IV SCOPE. Maximum Pressure to Earth or between Conductors of Un- earthed Systems, System. volts. I. The disposition of permanently A.C. 250 earthed power conductors * so that C.C. 650 they act also as guard wires. II. The use, for potential conductors, of A.C. 250 wires insulated with an approved C.C. 650 weatherproof covering f (bare wires being permitted for permanently earthed conductors). III. The use of any form of covered power A.C. or C.C. 650 conductors (including lead-covered cables) supported by earthed bare suspending wires (including neutral conductors). IY. The use by the Post Office of insulated A.C. 250 conductors for the telegraphs, other C.C. 650 than the telephone trunk circuits. V. The provision of independent guard A.C. or C.C. 650 wires. 5. The details of the normal requirements, which in some cases depend on the design of the power circuit and its position relatively to the telegraphs, are set forth in the following pages ; but, occa- sionally, e.g. in very exposed situations, it is necessary to impose more stringent requirements. 6. In all cases, i.e. irrespective of the pressure of the power circuit, a clearance of 4 feet should normally be given, but where it would be " difficult or costly to provide more than 3 feet this will be agreed. Further, at crossings a clearance of 2 feet will be agreed, at the discretion of the Post Office Sectional Engineer, in cases where the power line supports are placed in such a position with respect to Post Office wires where there will be no appreciable difference in sag with changes of temperature or under the worst conditions of ice loading, and that there is no danger to men working on the P.O. poles. If the P.O. line is not fully developed additional clearance * The term " earthed (power) conductor " as used in this Memorandum means " permanently earthed " and includes " neutral conductor." t Other braided conductors, with or without rubber, fall under arrangements I., III. and V. OVERHEAD POWER LINES may be necessary at the outset to provide for the ultimate con- ditions. 7. Telegraphs endangered by a power circuit are equipped with internal protective devices, i.e. fuses and heat coils, and the erection of a power circuit may involve the provision of such devices. 8. When the necessity for protection arises from the erection of telegraphs, i.e. the telegraphs are " second comer," the Postmaster- General as a concession will bear the cost of any fuses and heat coils installed and also the cost of the most economical method of guarding having regard to the ultimate conditions ; but in all other cases the whole cost of protection falls upon the undertaker. Power Power T f 1 T 1 i f 7 f M 1 f f t t t t t i t.. t M i i h GUARD WHEN "a" is LESS THAN "b". FIG. 1. (I) Permanently Earthed Conductors arranged to act as Guard Wires. 9. The earthed conductor shall consist of copper wire, not lighter than No. 11|- S.W.Gr., which shall be earthed at one point only, that is, at the generating station, sub-station, or transformer. The earth connection must be permanent and the electrical continuity of the wire must be maintained at all times. (A) TELEGRAPHS ABOVE A POWER CIRCUIT. (i} Crossings at Angles greater than 30. 10. An earthed conductor shall be erected above the potential conductors Figs. 2, 3 and 4. 11. "Where a vertical formation is used for the power wires as in APPENDIX IV Fig. 2 an earthed conductor shall be erected above the potential conductors in the same vertical plane with a minimum clearance of 8 inches to the highest potential wire. 12. Where the power wires are erected with any other than the vertical formation an earthed conductor shall be erected uppermost in such a position that lines drawn from it to the outermost potential wires will not make a greater angle than 45 with the vertical. See Fis. 3 and 4. Co/jc/uc&or* W/re Potent /a/ Conductor W/re. Pro. 3. PIG. 2. FIG. 4. ('ii) Crossings at Angles less than 30 <m$ Parallel Lines. 13. Where the angle of crossing is less than 30 two earthed conductor wires shall be provided if in the opinion of the Post Office Sectional Engineer the conditions as regards danger are not satis- factorily met by the provision of one earth guard wire. Figs. 5, 6, 7 and 8. Cross-lacing may also be required if there is direct overhanging, and "this should be provided by means of wire placed at intervals of 15 226 OVERHEAD POWER LINES not more than 6 feet for such a distance as may be stipulated by the Post Office Sectional Engineer. (B) TELEGRAPHS BELOW A POWER CIRCUIT. (i) Crossings at, or approximately at, Right Angles. 14. The earthed conductors shall be erected as shown in Fig. 9, which indicates three variations in their position relatively to the potential conductors. Conductor W/re . Potent /a/ Conductor fY/re >^ ^, ---- M/n.8' M/n.8" Min.8"\ \M P . ^.; FIG. C. \Afin. I \8" i y i FIG. 7. FIG. S. 15. (1) When the earthed conductors are erected below any "potential conductor, including a switch wire, the overlap a shall be greater than the vertical distance 6 between the highest potential conductor and the plane of the earthed conductors. (2) In all cases the earthed conductors shall be connected by cross wires of the same gauge passing under the potential conductors, the cross wires being spaced at intervals of not more than 6 feet for a distance of 18 feet on each side of the crossing, except where there is a pole within 18 feet, in which event the cross wires need not extend beyond the pole. APPENDIX IV 227 (ii) Diagonal Crossings and Overhangings. 16. Fig. 9 applies, "but tlie earthed conductors will be cross-laced at intervals of 6 feet, for such a distance as may be stipulated by the Post Office Sectional Engineer. (Hi] Parallel Lines. 17. Fig. 9 applies, but the earthed conductors will be cross-laced at intervals of 6 feet throughout the section affected, i.e. falling within the scope of paragraph 4 (b). A. A ,'* cross /actn n J cross lacino a to bt greater llum b r r- i ^ Telegraphs bghuj ^-^ eufral A 4- 4 k 4-6/1- -> t, cross -lac/na eutral FIG. 9. (II) Power Conductors Insulated with Approved Weatherproof Covering. 18. Conductors covered with a satisfactory weatherproof in- sulating material similar to that used by the Post Office at power crossings see (IV) below will be accepted as affording adequate protection, provided that the pressure to earth of the power circuit does not exceed 250 volts A.C. or 650 volts C.C. Further, within these limits of pressure, bare wire may be used for permanently earthed conductors. 228 OVERHEAD POWER LINES The covered conductor must withstand specified electrical tests of a searching character. These tests, together with the significant clauses in the Post Office specification for the type of covered conductor referred to are given in Appendix I. Conductors covered with this type of insulation can be obtained from the leading British makers of insulated wire and, if purchased to the specification from manufacturers approved by the Post Office, will be accepted as satisfactory, subject to the proviso that the Postmaster-General reserves the right in any par- ticular case to ascertain by tests on samples of the wire whether the specification is being complied with. Should the tests on the samples show that the wire is not to standard, approval of its use will be withheld. Varnished cambric will be accepted as an alternative to paper in the make up of the insulating covering. (Ill) Covered Power Conductors Supported by Earthed Bare Suspending Wires. 19. Any covered power conductor will be accepted as satis- factorily guarded if it is suspended from an efficiently earthed bare wire, by means of uninsulated ties. The distance between such ties shall not exceed 2 feet, and the suspending wire shall be earthed at both ends and, if necessary, at intermediate points, so that the dis- tance between any two earth connections does not exceed 200 yards. (IV) Insulated Conductors for the Telegraphs. 20. "When the pressure of the power circuit does not exceed 250 volts A.C. or 650 volts C.C., the Postmaster-den eral is prepared to erect or substitute insulated wire, instead of bare wire, for the tele- graphs, provided that a small number of telegraph wires is concerned, the route is not likely to grow, and that the efficiency of the circuits will not be impaired. Generally, the arrangement is not applicable to Post Office lines carrying telephone trunk circuits. (V) Independent Guard Wires. 21 . Guard wires should be, in general, of galvanised steel, mini- m-am gauge No. 8 S/W.G. or 7/16 S.W.G., but in manufacturing dis- tricts where such wire is liable to rapid corrosion, bronze or hard drawn copper wires of equivalent strength should be used. The supports for the guard wires should be rigid and of sufficient strength for their purpose, and at each support each guard wire should be securely bound in or terminated. APPENDIX IV 229 Each guard wire, including all the longitudinal -wires forming a cradle guard, must be well earthed at both ends, and at intervals .of not more than 200 yards. Each Earth should be made by means of a permanent connection to a water main, or by means of a substantial cast-iron earth plate fitted with a strand of copper wire of sufficient length to admit of the joint being made above ground. When first erected the resist- Earthed Guard W/re. @ Potential Wire. t ! %' 8 XL \ \ FIG. 10. /TV /> Mm. FIG, ance to earth of the guard wires should be tested, and periodical tests should be made by the undertaker to prove that each earth connection is efficient. (A) TELEGRAPHS ABOVE A POWER CIECTJIT. (?) Crossings at Angles greater than 30. 22 Where a vertical formation is used for the power wires, as in Fig. 10, a guard shall be erected above the power wires m the same 230 OVERHEAD POWER LINES vertical plane with, a minimum clearance of 8 inches to the highest power wire. 23. Where the power wires are erected with any other than a vertical formation a guard wire shall be erected uppermost in such a position that lines drawn from it to the outermost potential wires will not make greater angles than 45 with the vertical plane. See Figs. 11 and 12. >_- > M/r>.8* M/n.8" 8 Via. 13. FIG. 14. * >i / X >// lt> M/n.o P ' l \Afrn. -M/n.8" Af/n.8" M/n. FIG. 15. FIG. 16. (ii) Crossings at Angles less than 30 and Overhangings. 24. Where the angle of crossing is less than 30 two earthed guard wires shall be provided if in the opinion of the Post Office Sectional Engineer the conditions as regards danger are not satisfactorily met by the provision of one earthed guard wire. Figs. 13, 14, 15 and 16. Cross-lacing may also he required if there is direct overhanging, and this should be provided by means of wire placed at intervals of not more than 6 feet for such a distance as may be stipulated by the Post Office Sectional Engineer. (B) TELEGRAPHS BELOW A POWER CIRCUIT. (i) Crossings at, or approximately at, Right Angles. 25. An earthed cradle guard (Fig. 17) shall be erected between the power circuit supports. APPENDIX IV (u) Diagonal Crossings and Overhanging s. 26. Fig. 17 applies, but the cradle guard will be provided for such, a distance as may be stipulated by the Post Office Sectional Engineer. (m) Parallel Lines. 27. Fig. 17 applies, but the cradle guard will be provided through- out the section affected, i.e. falling within the scope of paragraph 4(6). mm 4 feet mesh not to exceed 2 feet x S feet X x x x .erTeleqrapbs xxx x J ' Fro. 17. APPENDIX I. SIGNIFICANT CLAUSES 01? THE P.O. SPECIFICATION FOR CONDUCTORS INSULATED WITH APPROVED WEATHERPROOF COVERING. GENERAL. The completed wire shall consist of a hard drawn copper wire insulated with two layers of impregnated paper covered with a layer of cotton lappings and o ottou braiding impregnated with a weatherproof composition. ' DIELECTRIC! AND WEATHERPROOF COVERING. Thetwc'. . ' "!!"" . ' . "i 1 * v.y- '' .'i 'i j. .i .: hi opposite directions ' : ." '!'... .y ' i r i. |.: . : .. , oroximately 5 turns in 3 inches. The paper shall be manilla of approximately -280 inch wide and -006 inch thick, treated with linseed oil. The cotton lappings and braiding shall be thoroughly impregnated with a mixture composed of approximately : Red Lead . . . . .72 parts by weight. Linseed Oil ](> Paraffin Wax , . . . ]2 The paraffin wax before being used shall be rendered anhydrous by being heated to a temperature of 300 to 350 F. until all water is expelled. The completed wire shall be finally passed through a bath of anhydrous paraffin wax at a temperature of 150 to 200 F,, so that the covering is left with a fairly smooth and glossy surface. 32 OVERHEAD POWER LINES INSULATION TESTS. The completed wire shall pass the following tests not less than 14 days after anufacture : (a) A piece of tinfoil 6 inches in length will be lapped closely round the wire at any points selected by the Inspecting Officer. An insulation test made between the conductor and the tinfoil, using 1 000 volts for the test, shall give a resistance of not less than 100 megolims. (b) A similar test to (a), made after the coil has been immersed in water for 24 hours, shall give a minimum resistance of 2 megohms. (c) Insulation tests will be made between bare wire closely lapped round the exterior of the completed wire and similar bare wire lappings inches distant. Three laps of the bare wire will be made at each point. The insula- tion shall be not less than 10 000 megohms when tested with I 000 volts. (a!) A similar test to (c), made after the coil has been immersed in wa.ter for 24 hours, shall give a minimum resistance of 100 megohms. 32,95 G-ENERAL INDEX. 'A' POLES, calculations for founda- tions, 120. scarf joint, 122. foil nd ations, ordinary type, 127. Anchora type, 131. Rutter type, 131. method of construction, 120. strength of, 113. Aluminium conductors, 185. steel conductors, 188. Anchorages, 17(5. Anchora typo of " A " polo foundations, 131. Angle of reposo of soil, 179. swing of conductors, 42, IOC. Angles in lino, 58, 103. properties of steel, 71. Arms, cross, oak, channel and angle, 70. Armstrong, Addison & Co., viii. Arrangement of conductors on polo, 44. Association, British Engineering Stand- ards, vii. AKELISED wood, 70. Basalt, fuzed, 09. Base plates, angle polos and struts, 108, 172. iron poles, 139. Basic loading conditions, conductors, 10. poles, 05, Bending moment on pole, 95. Binding in, 64. wire, approximate quantities required, 60. Bird guards, 41 . Blocks, brace, 127. foundation, 111, 110. kicking, 128. scarf, 123. stay, 170. Bolts, particulars of, 75. Bonding, 103, 194. Brace, blocks, 127. Brackets, insulator, 45-8, 87, 91. British Engineering Standards Associa- tion, vii. Bronze conductors, 1R3. Buckling strength of poles and struts, 118, 107, 172. c. CADMIUM copper conductors, 185. Oallender's standard polo fittings, 48. Cap and pin tensioning insulators, 08. Catenary curve, 18. Cementing insulator on to pin, 01. Channel iron cross arms, 78. polos, 140. Chart for determining economical span, .101. Choice of working voltage, 2. Clearance between conductors, 40. Clips, conductor, for pin insulators, 03. Coach screws, 70. Cobra impregnnting solution, 04. Coefficients of linear expansion, 184, self -induction, 2. Cohesion of soil, 1.70. Comparison between copper and alumi- nium, 180, 187. other materials, 184. stool, 191. Compound channel iron poles, 144. wood poles, 115. Concrete, allowable stress, 144, 154. composition, 144, 154. foundation 1 ?, 143. poles, 154. Conductors, aluminium, 185, steel-cored, 1F8. arrangement of, on polo, 4.4, bronze, 183. '.-: . 185". copper, sag and stress calculations, 18. particulars of, 4, critical temperature, 20. erection sags, H.V. lines, copper, 24. L.V. lines, copper, 20. tensions, H.V. lines, copper, 25. L.V. lines, copper, 27. sags and tensions, steel, 190. solid t. stranded, 37. spacing, 42, 234 OVERHEAD POWER LINES Conductors, steel, 190. Const-ruction of " A " pole. 120. Costs of supports, 103. Creosote i 1 ' *'--. Oi. Critical i . ' . Cross arms, 70. Crossings, railways, 197. roads and canals, 197. telegraph and telephone wires, 198. Cubic equations, 23. Curves. See Table XL D, 'ANGER notices, 207. Deflection of single wood poles, ] 13. steel tubular poles, 140. Depth of pole foundations, 109. Device, earthing, 196, 197, 205, 207. Dimensions of standard poles, 97. Displacement of conductors in wind, 35. Distribution, high voltage, 1. low voltage, 16. Double insulators, 49, 196, 197. Dry spark over voltages, 56. Duplicate insulators, 49, 196, 197. Dynamometer, 38. IARTHED neutral conductor, 194. Earthing device, 19C, 197. Earbhs, particulars of various, 179. Earth wires, 193. Ebonestos insulators, 68. Eccentric loading, 72. Economical span length, 100. Economy due to steel conductors, 190. Elastic extension and contraction of con- ductors, 20 ei seq. Elasticity, moduli of, 186. Electricity Commissioners' Regulations, 201. Energy loss in conductors, 9. Erection sags, copper conductors, 24, 26. and tensions, steel conductors, 190. tensions, copper conductors, 25, 27. Euler's formulae for struts, 74, 118, 167. Expansion, coefficients of linear, 186. JD ACTORS of safety, 203, 204. Ferro- concrete poles, 154. Fir poles, 94. Fittings, pole, calculations, 70. types of, 44. Flash over voltage, insulators, 56. Flexibility of poles, 114. Foundations, " A " poles, 126. Anchora type, 131. Rutier type, 131. Foundations, iron poles, 13S, 143, .149. Rutter poles, 116. single wood pole.-i, 105. Frequency of line vibrations, 43. supply, 3. Fuzed basalt, 69. G TALVANISED stool conductors, 190. stay wire, ] 70. Glass insulators, 68. Glazing of insulators, 55. Graphic solution of cubic equations, 23. Gripper for shackle insulator, 1(54. stay, 104. Guarding in general, 193. Guards, bird, 41 . climbing, 207. wire, 48, 53, 196, 197. H __ POLES, 133, 174. Height of conductors and wires from ground, 40. Hewlett tensioning insulator, 67. High voltage lines, basic loading con- ditions, 19. erection sags and tensions, 20. " ' -. " 'ions, .194, 195. of conductors in wind, 35. loading on conductors, 20. spacing of conductors, 42. I, CE loading, 20. Impregnation of poles, 94. Inductance, 2. Insulated hard drawn copper con- ductors, 198. Insulators, pin s at angles, 58. attachment of conductor, 62. to pin, 61. electrical design, 55. mechanical design, 56. standard tests, 56. stay, 194. tensioning, 66. Iron and steel conductors, 190. poles, 135. Ironwork, pole, 44, 70. JL\ ALANITE insulators, 68. Kicking blocks, 128. L: . INK type tensioning insulators, 67. Losses, energy in conductors, 9. Low voltage lines, basic loading condi- tions, 19. GENERAL INDEX Low voltage lines, erection sags an tensions, 20. safety precautions, 193, ] 94. AKING off stay wire on pole, 171 thimble, 170. Metal cap tension ing insulators, (18. Moduli of elasticity, concrete, 156. conductors, 184. steel and wood, 73. steel sections, 71. Modulus of rupture, red fir, 95. Moments of inertia of steel sections, 71. Most economical span, 100. ATURAL frequency of conducto swings, 42. Neutral conductor, L.V. and M.Y systems, 1 94, 205. "Norwegian red fir poles, 94. Number plates for poles, 207. VERTURNING moment, poles, 109 _L ARABOLA, 18. Pern ax insulating material, 42. Physical constants of conductor mate- rials, 184. Pins, insulator, 56. Pin type insulators, 56. Plaster of Paris, 62. Plates, base for iron poles, 139. danger, 207. earth, 194, 206. number, 207. stay, 182. Poles, ferro-concreto, 154. fittings, 44, 70. foundations, 105, 120, 138. iron, channel, 141. tubular, 135. ironwork, 44, 70.. wood, simple, 94. twin, 115. " A " tvpe, 116. " H " type, 133. _ Rutter type, 116. Porcelain insulators, 55. Post office regulations, 219. technical instructions, vi. Power factor, correction, 10. effect on line losses, 9. voltage regulation, 8. Precautions, safety, 193. Puncture voltage, insulators, 56. _O ADII of gyration of steel sections 71. Railway crossings, 1 97. Rake, at angle poles, 169. Reactance, 3. Regulation of sag of conductors, 38. voltage, 1. Regulations, Electricity Commissioners, 201. Post office, 219. Reinforced concrete poles, 1 54. Repose of soil, angles of, 179. Road crossings, 1 90. Rods, stay, 170, Rupture intensity of soil, 108, 181. P -'!-. " paint, 72. " A " pole fotvndation, 181. AFETY precautions, 103. Sag adjustment, sighting, 88. curves, 29,- ,'50, 31. by frequency of swings, 42. dynamometer, 3<S. Sag-tcmpcraturc calculations, 18. Sap wood, 70. Scarf joint, 122. Screws, coach, 70. Sections, steel, particulars of, 71. Self-induction, 2. Service lines, 194. Shackles, 65, 66. Shropshire, S. and W. Elec. Power Co., 52. Single-phase voltage drop, 7, Skin effect, 2, 192. Soil, particulars of various, .179. Spacing of conductors, horizontal, 42. vertical, 44. Specifications, Britisli Standard, vii. Standard poles, 97. span lengths, 104. specifications, vii. "Stay Mocks, 176. insulators, 68. rods, 170. wires, 170. attachment to pole and thimble, 170, 171. teatite insulators, 68. teel conductors, 190. cross arms, 78. poles, channel, 140. tubular, 136. sections, standard, particulars of, 71. truts, 166. ynohronous swinging of wires, 42. 236 OVERHEAD POWER LINES J_ ABLES. flee list, x. Tackle, trussing for "II " poles, 174 Taper of wood poles, 98. TeJenduron insulators, OS. Telephone and telegraph crossings, 198, 219. Temperature, critical, 20. Temperature-sag calculations, IS. Tensioning insulators, GO. Terminal poles, 1(>9. Terminating conductors, GO. Three-phase voltage drop, 7. Tolerance, voltage, 1. Transmission, energy loss in, 0, 12, 13. voltage drop in, S, 11. Trussing tackle for " JT " pole, 174. Tubular steel poles, 136. Twiss ten sinning insulator, 67. u LTIMATE stresses of materials, 73. V GUARDS, 52. Variation of reactance with spacing, 3. Vector diagram of voltages, l>. Vertical spacing between conductors, 44. Voltage, automatic regulation, 1. choice of working values, 2. regulation, 5. tolerances, 1. ADE, Gabriel & English, viii. Wayleaves, v, 103. Weights, cement and concrete, 144, 154. conductor materials, 184. insulators, 57. steel sections, 71. various soils, 1 79. wood poles, 97. Wet spark over distances, insulators, 50, 57. Wire guards, 48-53, 195. Wood cross arms, 70. polos, compound Butter type, 110. " A " type, 116. "H"type, 133. simple, 94. standard sizes, 97. Working stresses, bolts, 75. soil, 181. steel. 74. timber, 76, \ \ TRUSTED IK (IRMAT BRITAIN HY THE ABERDEEN UNIVERSITY KRESS, ABERDEEN